path: root/docs/build/tutorial.rst

========
Tutorial
========

`Alembic <http://bitbucket.org/zzzeek/alembic>`_ provides for the creation, management, and invocation of *change management*
scripts for a relational database, using `SQLAlchemy <http://www.sqlalchemy.org>`_ as the underlying engine.
This tutorial will provide a full introduction to the theory and usage of this tool.

To begin, make sure Alembic is installed as described at :ref:`installation`.

The Migration Environment
==========================

Usage of Alembic starts with creation of the *Migration Environment*.  This is a directory of scripts
that is specific to a particular application.   The migration environment is created just once,
and is then maintained along with the application's source code itself.   The environment is
created using the ``init`` command of Alembic, and is then customizable to suit the specific
needs of the application.

The structure of this environment, including some generated migration scripts, looks like::

    yourproject/
        alembic/
            env.py
            README
            script.py.mako
            versions/
                3512b954651e_add_account.py
                2b1ae634e5cd_add_order_id.py
                3adcc9a56557_rename_username_field.py

The directory includes these directories/files:

* ``yourproject`` - this is the root of your application's source code, or some directory within it.
* ``alembic`` - this directory lives within your application's source tree and is the home of the
  migration environment.   It can be named anything, and a project that uses multiple databases
  may even have more than one.
* ``env.py`` - This is a Python script that is run whenever the alembic migration tool is invoked.
  At the very least, it contains instructions to configure and generate a SQLAlchemy engine,
  procure a connection from that engine along with a transaction, and to then invoke the migration
  engine, using the connection as a source of database connectivity.

  The ``env.py`` script is part of the generated environment so that the way migrations run
  is entirely customizable.   The exact specifics of how to connect are here, as well as
  the specifics of how the migration enviroment are invoked.  The script can be modified
  so that multiple engines can be operated upon, custom arguments can be passed into the
  migration environment, application-specific libraries and models can be loaded in and
  made available.

  Alembic includes a set of initialization templates which feature different varieties
  of ``env.py`` for different use cases.
* ``README`` - included with the various enviromnent templates, should have something
  informative.
* ``script.py.mako`` - This is a `Mako <http://www.makotemplates.org>`_ template file which
  is used to generate new migration scripts.   Whatever is here is used to generate new
  files within ``versions/``.   This is scriptable so that the structure of each migration
  file can be controlled, including standard imports to be within each, as well as
  changes to the structure of the ``upgrade()`` and ``downgrade()`` functions.  For example,
  the ``multidb`` environment allows for multiple functions to be generated using a
  naming scheme ``upgrade_engine1()``, ``upgrade_engine2()``.
* ``versions/`` - This directory holds the individual version scripts.  Users of other migration
  tools may notice that the files here don't use ascending integers, and instead use a
  partial GUID approach.   In Alembic, the ordering of version scripts is relative
  to directives within the scripts themselves, and it is theoretically possible to "splice" version files
  in between others, allowing migration sequences from different branches to be merged,
  albeit carefully by hand.


Creating an Environment
=======================

With a basic understanding of what the environment is, we can create one using ``alembic init``.
This will create an environment using the "generic" template::

    $ cd yourproject
    $ alembic init alembic

Where above, the ``init`` command was called to generate a migrations directory called ``alembic``::

    Creating directory /path/to/yourproject/alembic...done
    Creating directory /path/to/yourproject/alembic/versions...done
    Generating /path/to/yourproject/alembic.ini...done
    Generating /path/to/yourproject/alembic/env.py...done
    Generating /path/to/yourproject/alembic/README...done
    Generating /path/to/yourproject/alembic/script.py.mako...done
    Please edit configuration/connection/logging settings in
    '/path/to/yourproject/alembic.ini' before proceeding.

Alembic also includes other environment templates.  These can be listed out using the ``list_templates``
command::

    $ alembic list_templates
    Available templates:

    generic - Generic single-database configuration.
    multidb - Rudimentary multi-database configuration.
    pylons - Configuration that reads from a Pylons project environment.

    Templates are used via the 'init' command, e.g.:

      alembic init --template pylons ./scripts

Editing the .ini File
=====================

Alembic placed a file ``alembic.ini`` into the current directory.  This is a file that the ``alembic``
script looks for when invoked.  This file can be anywhere, either in the same directory
from which the ``alembic`` script will normally be invoked, or if in a different directory, can
be specified by using the ``--config`` option to the ``alembic`` runner.

The file generated with the "generic" configuration looks like::

    # A generic, single database configuration.

    [alembic]
    # path to migration scripts
    script_location = alembic

    # template used to generate migration files
    # file_template = %%(rev)s_%%(slug)s

    # max length of characters to apply to the
    # "slug" field
    #truncate_slug_length = 40

    # set to 'true' to run the environment during
    # the 'revision' command, regardless of autogenerate
    # revision_environment = false

    # set to 'true' to allow .pyc and .pyo files without
    # a source .py file to be detected as revisions in the
    # versions/ directory
    # sourceless = false

    # the output encoding used when revision files
    # are written from script.py.mako
    # output_encoding = utf-8

    sqlalchemy.url = driver://user:pass@localhost/dbname

    # Logging configuration
    [loggers]
    keys = root,sqlalchemy,alembic

    [handlers]
    keys = console

    [formatters]
    keys = generic

    [logger_root]
    level = WARN
    handlers = console
    qualname =

    [logger_sqlalchemy]
    level = WARN
    handlers =
    qualname = sqlalchemy.engine

    [logger_alembic]
    level = INFO
    handlers =
    qualname = alembic

    [handler_console]
    class = StreamHandler
    args = (sys.stderr,)
    level = NOTSET
    formatter = generic

    [formatter_generic]
    format = %(levelname)-5.5s [%(name)s] %(message)s
    datefmt = %H:%M:%S

The file is read using Python's :class:`ConfigParser.SafeConfigParser` object.  The
``%(here)s`` variable is provided as a substitution variable, which
can be used to produce absolute pathnames to directories and files, as we do above
with the path to the Alembic script location.

This file contains the following features:

* ``[alembic]`` - this is the section read by Alembic to determine configuration.  Alembic
  itself does not directly read any other areas of the file.
* ``script_location`` - this is the location of the Alembic environment.   It is normally
  specified as a filesystem location, either relative or absolute.  If the location is
  a relative path, it's interpreted as relative to the current directory.

  This is the only key required by Alembic in all cases.   The generation
  of the .ini file by the command ``alembic init alembic`` automatically placed the
  directory name ``alembic`` here.   The special variable ``%(here)s`` can also be used,
  as in ``%(here)s/alembic``.

  For support of applications that package themselves into .egg files, the value can
  also be specified
  as a `package resource <https://pythonhosted.org/setuptools/pkg_resources.html>`_, in which
  case ``resource_filename()`` is used to find the file (new in 0.2.2).  Any non-absolute
  URI which contains colons is interpreted here as a resource name, rather than
  a straight filename.

* ``file_template`` - this is the naming scheme used to generate new migration files.
  The value present is the default, so is commented out.   Tokens available include:

    * ``%%(rev)s`` - revision id
    * ``%%(slug)s`` - a truncated string derived from the revision message
    * ``%%(year)d``, ``%%(month).2d``, ``%%(day).2d``, ``%%(hour).2d``,
      ``%%(minute).2d``, ``%%(second).2d`` - components of the create date
      as returned by ``datetime.datetime.now()``

* ``truncate_slug_length`` - defaults to 40, the max number of characters
  to include in the "slug" field.

  .. versionadded:: 0.6.1 - added ``truncate_slug_length`` configuration

* ``sqlalchemy.url`` - A URL to connect to the database via SQLAlchemy.  This key is in fact
  only referenced within the ``env.py`` file that is specific to the "generic" configuration;
  a file that can be customized by the developer. A multiple
  database configuration may respond to multiple keys here, or may reference other sections
  of the file.
* ``revision_environment`` - this is a flag which when set to the value 'true', will indicate
  that the migration environment script ``env.py`` should be run unconditionally when
  generating new revision files

* ``sourceless`` - when set to 'true', revision files that only exist as .pyc
  or .pyo files in the versions directory will be used as versions, allowing
  "sourceless" versioning folders.  When left at the default of 'false',
  only .py files are consumed as version files.

  .. versionadded:: 0.6.4

* ``output_encoding`` - the encoding to use when Alembic writes the
  ``script.py.mako`` file into a new migration file.  Defaults to ``'utf-8'``.

  .. versionadded:: 0.7.0

* ``[loggers]``, ``[handlers]``, ``[formatters]``, ``[logger_*]``, ``[handler_*]``,
  ``[formatter_*]`` - these sections are all part of Python's standard logging configuration,
  the mechanics of which are documented at `Configuration File Format <http://docs.python.org/library/logging.config.html#configuration-file-format>`_.
  As is the case with the database connection, these directives are used directly as the
  result of the ``logging.config.fileConfig()`` call present in the
  ``env.py`` script, which you're free to modify.

For starting up with just a single database and the generic configuration, setting up
the SQLAlchemy URL is all that's needed::

    sqlalchemy.url = postgresql://scott:tiger@localhost/test


.. _create_migration:

Create a Migration Script
=========================

With the environment in place we can create a new revision, using ``alembic revision``::

    $ alembic revision -m "create account table"
    Generating /path/to/yourproject/alembic/versions/1975ea83b712_create_accoun
    t_table.py...done

A new file ``1975ea83b712_create_account_table.py`` is generated.  Looking inside the file::

    """create account table

    Revision ID: 1975ea83b712
    Revises:
    Create Date: 2011-11-08 11:40:27.089406

    """

    # revision identifiers, used by Alembic.
    revision = '1975ea83b712'
    down_revision = None
    branch_labels = None

    from alembic import op
    import sqlalchemy as sa

    def upgrade():
        pass

    def downgrade():
        pass

The file contains some header information, identifiers for the current revision
and a "downgrade" revision, an import of basic Alembic directives,
and empty ``upgrade()`` and ``downgrade()`` functions.  Our
job here is to populate the ``upgrade()`` and ``downgrade()`` functions with directives that
will apply a set of changes to our database.    Typically, ``upgrade()`` is required
while ``downgrade()`` is only needed if down-revision capability is desired, though it's
probably a good idea.

Another thing to notice is the ``down_revision`` variable.  This is how Alembic
knows the correct order in which to apply migrations.   When we create the next revision,
the new file's ``down_revision`` identifier would point to this one::

    # revision identifiers, used by Alembic.
    revision = 'ae1027a6acf'
    down_revision = '1975ea83b712'

Every time Alembic runs an operation against the ``versions/`` directory, it reads all
the files in, and composes a list based on how the ``down_revision`` identifiers link together,
with the ``down_revision`` of ``None`` representing the first file.   In theory, if a
migration environment had thousands of migrations, this could begin to add some latency to
startup, but in practice a project should probably prune old migrations anyway
(see the section :ref:`building_uptodate` for a description on how to do this, while maintaining
the ability to build the current database fully).

We can then add some directives to our script, suppose adding a new table ``account``::

    def upgrade():
        op.create_table(
            'account',
            sa.Column('id', sa.Integer, primary_key=True),
            sa.Column('name', sa.String(50), nullable=False),
            sa.Column('description', sa.Unicode(200)),
        )

    def downgrade():
        op.drop_table('account')

:meth:`~.Operations.create_table` and :meth:`~.Operations.drop_table` are Alembic directives.   Alembic provides
all the basic database migration operations via these directives, which are designed to be as simple and
minimalistic as possible;
there's no reliance upon existing table metadata for most of these directives.  They draw upon
a global "context" that indicates how to get at a database connection (if any; migrations can
dump SQL/DDL directives to files as well) in order to invoke the command.   This global
context is set up, like everything else, in the ``env.py`` script.

An overview of all Alembic directives is at :ref:`ops`.

Running our First Migration
===========================

We now want to run our migration.   Assuming our database is totally clean, it's as
yet unversioned.   The ``alembic upgrade`` command will run upgrade operations, proceeding
from the current database revision, in this example ``None``, to the given target revision.
We can specify ``1975ea83b712`` as the revision we'd like to upgrade to, but it's easier
in most cases just to tell it "the most recent", in this case ``head``::

    $ alembic upgrade head
    INFO  [alembic.context] Context class PostgresqlContext.
    INFO  [alembic.context] Will assume transactional DDL.
    INFO  [alembic.context] Running upgrade None -> 1975ea83b712

Wow that rocked!   Note that the information we see on the screen is the result of the
logging configuration set up in ``alembic.ini`` - logging the ``alembic`` stream to the
console (standard error, specifically).

The process which occurred here included that Alembic first checked if the database had
a table called ``alembic_version``, and if not, created it.   It looks in this table
for the current version, if any, and then calculates the path from this version to
the version requested, in this case ``head``, which is known to be ``1975ea83b712``.
It then invokes the ``upgrade()`` method in each file to get to the target revision.

Running our Second Migration
=============================

Let's do another one so we have some things to play with.    We again create a revision
file::

    $ alembic revision -m "Add a column"
    Generating /path/to/yourapp/alembic/versions/ae1027a6acf_add_a_column.py...
    done

Let's edit this file and add a new column to the ``account`` table::

    """Add a column

    Revision ID: ae1027a6acf
    Revises: 1975ea83b712
    Create Date: 2011-11-08 12:37:36.714947

    """

    # revision identifiers, used by Alembic.
    revision = 'ae1027a6acf'
    down_revision = '1975ea83b712'

    from alembic import op
    import sqlalchemy as sa

    def upgrade():
        op.add_column('account', sa.Column('last_transaction_date', sa.DateTime))

    def downgrade():
        op.drop_column('account', 'last_transaction_date')

Running again to ``head``::

    $ alembic upgrade head
    INFO  [alembic.context] Context class PostgresqlContext.
    INFO  [alembic.context] Will assume transactional DDL.
    INFO  [alembic.context] Running upgrade 1975ea83b712 -> ae1027a6acf

We've now added the ``last_transaction_date`` column to the database.

.. relative_migrations:

Relative Migration Identifiers
==============================

Relative upgrades/downgrades are also supported.  To move two versions from
the current, a decimal value "+N" can be supplied::

    $ alembic upgrade +2

Negative values are accepted for downgrades::

    $ alembic downgrade -1

Partial Revision Identifiers
=============================

Any time we need to refer to a revision number explicitly, we have the option
to use a partial number.  As long as this number uniquely identifies the
version, it may be used in any command in any place that version numbers
are accepted::

    $ alembic upgrade ae1

Above, we use ``ae1`` to refer to revision ``ae1027a6acf``.
Alembic will stop and let you know if more than one version starts with
that prefix.

Getting Information
===================

With a few revisions present we can get some information about the state of things.

First we can view the current revision::

    $ alembic current
    INFO  [alembic.context] Context class PostgresqlContext.
    INFO  [alembic.context] Will assume transactional DDL.
    Current revision for postgresql://scott:XXXXX@localhost/test: 1975ea83b712 -> ae1027a6acf (head), Add a column

``head`` is displayed only if the revision identifier for this database matches the head revision.

We can also view history with ``alembic history``; the ``--verbose`` option
(accepted by several commands, including ``history``, ``current``, ``heads``
and ``branches``) will show us full information about each revision::

    $ alembic history --verbose

    Rev: ae1027a6acf (head)
    Parent: 1975ea83b712
    Path: /path/to/yourproject/alembic/versions/ae1027a6acf_add_a_column.py

        add a column

        Revision ID: ae1027a6acf
        Revises: 1975ea83b712
        Create Date: 2014-11-20 13:02:54.849677

    Rev: 1975ea83b712
    Parent: <base>
    Path: /path/to/yourproject/alembic/versions/1975ea83b712_add_account_table.py

        add account table

        Revision ID: 1975ea83b712
        Revises:
        Create Date: 2014-11-20 13:02:46.257104

Viewing History Ranges
----------------------

Using the ``-r`` option to ``alembic history``, we can also view various slices
of history.  The ``-r`` argument accepts an argument ``[start]:[end]``, where
either may be a revision number, symbols like ``head``, ``heads`` or
``base``,  ``current`` to specify the current revision(s), as well as negative
relative ranges for ``[start]`` and positive relative ranges for ``[end]``::

  $ alembic history -r1975ea:ae1027

A relative range starting from three revs ago up to current migration,
which will invoke the migration environment against the database
to get the current migration::

  $ alembic history -r-3:current

View all revisions from 1975 to the head::

  $ alembic history -r1975ea:

.. versionadded:: 0.6.0  ``alembic revision`` now accepts the ``-r`` argument to
   specify specific ranges based on version numbers, symbols, or relative deltas.


Downgrading
===========

We can illustrate a downgrade back to nothing, by calling ``alembic downgrade`` back
to the beginning, which in Alembic is called ``base``::

    $ alembic downgrade base
    INFO  [alembic.context] Context class PostgresqlContext.
    INFO  [alembic.context] Will assume transactional DDL.
    INFO  [alembic.context] Running downgrade ae1027a6acf -> 1975ea83b712
    INFO  [alembic.context] Running downgrade 1975ea83b712 -> None

Back to nothing - and up again::

    $ alembic upgrade head
    INFO  [alembic.context] Context class PostgresqlContext.
    INFO  [alembic.context] Will assume transactional DDL.
    INFO  [alembic.context] Running upgrade None -> 1975ea83b712
    INFO  [alembic.context] Running upgrade 1975ea83b712 -> ae1027a6acf


Auto Generating Migrations
===========================

Alembic can view the status of the database and compare against the table metadata
in the application, generating the "obvious" migrations based on a comparison.  This
is achieved using the ``--autogenerate`` option to the ``alembic revision`` command,
which places so-called *candidate* migrations into our new migrations file.  We
review and modify these by hand as needed, then proceed normally.

To use autogenerate, we first need to modify our ``env.py`` so that it gets access
to a table metadata object that contains the target.  Suppose our application
has a `declarative base <http://www.sqlalchemy.org/docs/orm/extensions/declarative.html#synopsis>`_
in ``myapp.mymodel``.  This base contains a :class:`~sqlalchemy.schema.MetaData` object which
contains :class:`~sqlalchemy.schema.Table` objects defining our database.  We make sure this
is loaded in ``env.py`` and then passed to :meth:`.EnvironmentContext.configure` via the
``target_metadata`` argument.   The ``env.py`` sample script already has a
variable declaration near the top for our convenience, where we replace ``None``
with our :class:`~sqlalchemy.schema.MetaData`.  Starting with::

    # add your model's MetaData object here
    # for 'autogenerate' support
    # from myapp import mymodel
    # target_metadata = mymodel.Base.metadata
    target_metadata = None

we change to::

    from myapp.mymodel import Base
    target_metadata = Base.metadata

If we look later in the script, down in ``run_migrations_online()``,
we can see the directive passed to :meth:`.EnvironmentContext.configure`::

    def run_migrations_online():
        engine = engine_from_config(
                    config.get_section(config.config_ini_section), prefix='sqlalchemy.')

        connection = engine.connect()
        context.configure(
                    connection=connection,
                    target_metadata=target_metadata
                    )

        trans = connection.begin()
        try:
            context.run_migrations()
            trans.commit()
        except:
            trans.rollback()
            raise

We can then use the ``alembic revision`` command in conjunction with the
``--autogenerate`` option.  Suppose
our :class:`~sqlalchemy.schema.MetaData` contained a definition for the ``account`` table,
and the database did not.  We'd get output like::

    $ alembic revision --autogenerate -m "Added account table"
    INFO [alembic.context] Detected added table 'account'
    Generating /Users/classic/Desktop/tmp/alembic/versions/27c6a30d7c24.py...done

We can then view our file ``27c6a30d7c24.py`` and see that a rudimentary migration
is already present::

    """empty message

    Revision ID: 27c6a30d7c24
    Revises: None
    Create Date: 2011-11-08 11:40:27.089406

    """

    # revision identifiers, used by Alembic.
    revision = '27c6a30d7c24'
    down_revision = None

    from alembic import op
    import sqlalchemy as sa

    def upgrade():
        ### commands auto generated by Alembic - please adjust! ###
        op.create_table(
        'account',
        sa.Column('id', sa.Integer()),
        sa.Column('name', sa.String(length=50), nullable=False),
        sa.Column('description', sa.VARCHAR(200)),
        sa.Column('last_transaction_date', sa.DateTime()),
        sa.PrimaryKeyConstraint('id')
        )
        ### end Alembic commands ###

    def downgrade():
        ### commands auto generated by Alembic - please adjust! ###
        op.drop_table("account")
        ### end Alembic commands ###

The migration hasn't actually run yet, of course.  We do that via the usual ``upgrade``
command.   We should also go into our migration file and alter it as needed, including
adjustments to the directives as well as the addition of other directives which these may
be dependent on - specifically data changes in between creates/alters/drops.

Autogenerate will by default detect:

* Table additions, removals.
* Column additions, removals.
* Change of nullable status on columns.
* Basic changes in indexes and explcitly-named unique constraints

.. versionadded:: 0.6.1 Support for autogenerate of indexes and unique constraints.

Autogenerate can *optionally* detect:

* Change of column type.  This will occur if you set
  the :paramref:`.EnvironmentContext.configure.compare_type` parameter
  to ``True``, or to a custom callable.
  The feature works well in most cases,
  but is off by default so that it can be tested on the target schema
  first.  It can also be customized by passing a callable here; see the
  function's documentation for details.
* Change of server default.  This will occur if you set
  the :paramref:`.EnvironmentContext.configure.compare_server_default`
  paramter to ``True``, or to a custom callable.
  This feature works well for simple cases but cannot always produce
  accurate results.  The Postgresql backend will actually invoke
  the "detected" and "metadata" values against the database to
  determine equivalence.  The feature is off by default so that
  it can be tested on the target schema first.  Like type comparison,
  it can also be customized by passing a callable; see the
  function's documentation for details.

Autogenerate can *not* detect:

* Changes of table name.   These will come out as an add/drop of two different
  tables, and should be hand-edited into a name change instead.
* Changes of column name.  Like table name changes, these are detected as
  a column add/drop pair, which is not at all the same as a name change.
* Anonymously named constraints.  Give your constraints a name,
  e.g. ``UniqueConstraint('col1', 'col2', name="my_name")``
* Special SQLAlchemy types such as :class:`~sqlalchemy.types.Enum` when generated
  on a backend which doesn't support ENUM directly - this because the
  representation of such a type
  in the non-supporting database, i.e. a CHAR+ CHECK constraint, could be
  any kind of CHAR+CHECK.  For SQLAlchemy to determine that this is actually
  an ENUM would only be a guess, something that's generally a bad idea.
  To implement your own "guessing" function here, use the
  :meth:`sqlalchemy.events.DDLEvents.column_reflect` event
  to alter the SQLAlchemy type passed for certain columns and possibly
  :meth:`sqlalchemy.events.DDLEvents.after_parent_attach` to intercept
  unwanted CHECK constraints.

Autogenerate can't currently, but will *eventually* detect:

* Some free-standing constraint additions and removals,
  like CHECK and FOREIGN KEY - these are not fully implemented.
* Sequence additions, removals - not yet implemented.


.. _autogen_render_types:

Rendering Custom Types in Autogenerate
--------------------------------------

The methodology Alembic uses to generate SQLAlchemy type constructs
as Python code is plain old ``__repr__()``.   SQLAlchemy's built-in types
for the most part have a ``__repr__()`` that faithfully renders a
Python-compatible constructor call, but there are some exceptions, particularly
in those cases when a constructor accepts arguments that aren't compatible
with ``__repr__()``, such as a pickling function.

When building a custom type that will be rendered into a migration script,
it is often necessary to explicitly give the type a ``__repr__()`` that will
faithfully reproduce the constructor for that type.   But beyond that, it
also is usually necessary to change how the enclosing module or package
is rendered as well;
this is accomplished using the :paramref:`.EnvironmentContext.configure.render_item`
configuration option::

    def render_item(type_, obj, autogen_context):
        """Apply custom rendering for selected items."""

        if type_ == 'type' and isinstance(obj, MySpecialType):
            return "mypackage.%r" % obj

        # default rendering for other objects
        return False

    def run_migrations_online():
        # ...

        context.configure(
                    connection=connection,
                    target_metadata=target_metadata,
                    render_item=render_item,
                    # ...
                    )

        # ...

Above, we also need to make sure our ``MySpecialType`` includes an appropriate
``__repr__()`` method, which is invoked when we call it against ``"%r"``.

The callable we use for :paramref:`.EnvironmentContext.configure.render_item`
can also add imports to our migration script.  The ``autogen_context`` passed in
contains an entry called ``autogen_context['imports']``, which is a Python
``set()`` for which we can add new imports.  For example, if ``MySpecialType``
were in a module called ``mymodel.types``, we can add the import for it
as we encounter the type::

    def render_item(type_, obj, autogen_context):
        """Apply custom rendering for selected items."""

        if type_ == 'type' and isinstance(obj, MySpecialType):
            # add import for this type
            autogen_context['imports'].add("from mymodel import types")
            return "types.%r" % obj

        # default rendering for other objects
        return False

The finished migration script will include our imports where the
``${imports}`` expression is used, producing output such as::

  from alembic import op
  import sqlalchemy as sa
  from mymodel import types

  def upgrade():
      op.add_column('sometable', Column('mycolumn', types.MySpecialType()))

.. _autogen_module_prefix:

Controlling the Module Prefix
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

When using :paramref:`.EnvironmentContext.configure.render_item`, note that
we deliver not just the reproduction of the type, but we can also deliver the
"module prefix", which is a module namespace from which our type can be found
within our migration script.  When Alembic renders SQLAlchemy types, it will
typically use the value of
:paramref:`.EnvironmentContext.configure.sqlalchemy_module_prefix`,
which defaults to ``"sa."``, to achieve this::

    Column("my_column", sa.Integer())

When we use a custom type that is not within the ``sqlalchemy.`` module namespace,
by default Alembic will use the **value of __module__ for the custom type**::

    Column("my_column", myapp.models.utils.types.MyCustomType())

Above, it seems our custom type is in a very specific location, based on
the length of what ``__module__`` reports.   It's a good practice to
not have this long name render into our migration scripts, as it means
this long and arbitrary name will be hardcoded into all our migration
scripts; instead, we should create a module that is
explicitly for custom types that our migration files will use.  Suppose
we call it ``myapp.migration_types``::

  # myapp/migration_types.py

  from myapp.models.utils.types import MyCustomType

We can provide the name of this module to our autogenerate context using
:paramref:`.EnvironmentContext.configure.user_module_prefix`
option::


    def run_migrations_online():
        # ...

        context.configure(
                    connection=connection,
                    target_metadata=target_metadata,
                    user_module_prefix="myapp.migration_types.",
                    # ...
                    )

        # ...

Where we'd get a migration like::

  Column("my_column", myapp.migration_types.MyCustomType())

Now, when we inevitably refactor our application to move ``MyCustomType``
somewhere else, we only need modify the ``myapp.migration_types`` module,
instead of searching and replacing all instances within our migration scripts.

.. versionchanged:: 0.7.0
   :paramref:`.EnvironmentContext.configure.user_module_prefix`
   no longer defaults to the value of
   :paramref:`.EnvironmentContext.configure.sqlalchemy_module_prefix`
   when left at ``None``; the ``__module__`` attribute is now used.

.. versionadded:: 0.6.3 Added :paramref:`.EnvironmentContext.configure.user_module_prefix`.


Generating SQL Scripts (a.k.a. "Offline Mode")
==============================================

A major capability of Alembic is to generate migrations as SQL scripts, instead of running
them against the database - this is also referred to as *offline mode*.
This is a critical feature when working in large organizations
where access to DDL is restricted, and SQL scripts must be handed off to DBAs.   Alembic makes
this easy via the ``--sql`` option passed to any ``upgrade`` or ``downgrade`` command.   We
can, for example, generate a script that revises up to rev ``ae1027a6acf``::

    $ alembic upgrade ae1027a6acf --sql
    INFO  [alembic.context] Context class PostgresqlContext.
    INFO  [alembic.context] Will assume transactional DDL.
    BEGIN;

    CREATE TABLE alembic_version (
        version_num VARCHAR(32) NOT NULL
    );

    INFO  [alembic.context] Running upgrade None -> 1975ea83b712
    CREATE TABLE account (
        id SERIAL NOT NULL,
        name VARCHAR(50) NOT NULL,
        description VARCHAR(200),
        PRIMARY KEY (id)
    );

    INFO  [alembic.context] Running upgrade 1975ea83b712 -> ae1027a6acf
    ALTER TABLE account ADD COLUMN last_transaction_date TIMESTAMP WITHOUT TIME ZONE;

    INSERT INTO alembic_version (version_num) VALUES ('ae1027a6acf');

    COMMIT;


While the logging configuration dumped to standard error, the actual script was dumped to standard output -
so in the absence of further configuration (described later in this section), we'd at first be using output
redirection to generate a script::

    $ alembic upgrade ae1027a6acf --sql > migration.sql

Getting the Start Version
--------------------------

Notice that our migration script started at the base - this is the default when using offline
mode, as no database connection is present and there's no ``alembic_version`` table to read from.

One way to provide a starting version in offline mode is to provide a range to the command line.
This is accomplished by providing the "version" in ``start:end`` syntax::

    $ alembic upgrade 1975ea83b712:ae1027a6acf --sql > migration.sql

The ``start:end`` syntax is only allowed in offline mode; in "online" mode, the ``alembic_version``
table is always used to get at the current version.

It's also possible to have the ``env.py`` script retrieve the "last" version from
the local environment, such as from a local file.   A scheme like this would basically
treat a local file in the same way ``alembic_version`` works::

    if context.is_offline_mode():
        version_file = os.path.join(os.path.dirname(config.config_file_name), "version.txt")
        if os.path.exists(version_file):
            current_version = open(version_file).read()
        else:
            current_version = None
        context.configure(dialect_name=engine.name, starting_version=current_version)
        context.run_migrations()
        end_version = context.get_revision_argument()
        if end_version and end_version != current_version:
            open(version_file, 'w').write(end_version)

Writing Migration Scripts to Support Script Generation
------------------------------------------------------

The challenge of SQL script generation is that the scripts we generate can't rely upon
any client/server database access.  This means a migration script that pulls some rows
into memory via a ``SELECT`` statement will not work in ``--sql`` mode.   It's also
important that the Alembic directives, all of which are designed specifically to work
in both "live execution" as well as "offline SQL generation" mode, are used.

Customizing the Environment
---------------------------

Users of the ``--sql`` option are encouraged to hack their ``env.py`` files to suit their
needs.  The ``env.py`` script as provided is broken into two sections: ``run_migrations_online()``
and ``run_migrations_offline()``.  Which function is run is determined at the bottom of the
script by reading :meth:`.EnvironmentContext.is_offline_mode`, which basically determines if the
``--sql`` flag was enabled.

For example, a multiple database configuration may want to run through each
database and set the output of the migrations to different named files - the :meth:`.EnvironmentContext.configure`
function accepts a parameter ``output_buffer`` for this purpose.  Below we illustrate
this within the ``run_migrations_offline()`` function::

    from alembic import context
    import myapp
    import sys

    db_1 = myapp.db_1
    db_2 = myapp.db_2

    def run_migrations_offline():
        """Run migrations *without* a SQL connection."""

        for name, engine, file_ in [
            ("db1", db_1, "db1.sql"),
            ("db2", db_2, "db2.sql"),
        ]:
            context.configure(
                        url=engine.url,
                        transactional_ddl=False,
                        output_buffer=open(file_, 'w'))
            context.execute("-- running migrations for '%s'" % name)
            context.run_migrations(name=name)
            sys.stderr.write("Wrote file '%s'" % file_)

    def run_migrations_online():
        """Run migrations *with* a SQL connection."""

        for name, engine in [
            ("db1", db_1),
            ("db2", db_2),
        ]:
            connection = engine.connect()
            context.configure(connection=connection)
            try:
                context.run_migrations(name=name)
                session.commit()
            except:
                session.rollback()
                raise

    if context.is_offline_mode():
        run_migrations_offline()
    else:
        run_migrations_online()

.. _batch_migrations:

Running "Batch" Migrations for SQLite and Other Databases
=========================================================

.. note:: "Batch mode" for SQLite and other databases is a new and intricate
   feature within the 0.7.0 series of Alembic, and should be
   considered as "beta" for the next several releases.

.. versionadded:: 0.7.0

The SQLite database presents a challenge to migration tools
in that it has almost no support for the ALTER statement upon which
relational schema migrations rely upon.  The rationale for this stems from
philosophical and architectural concerns within SQLite, and they are unlikely
to be changed.

Migration tools are instead expected to produce copies of SQLite tables that
correspond to the new structure, transfer the data from the existing
table to the new one, then drop the old table.  For our purposes here
we'll call this **"move and copy"** workflow, and in order to accommodate it
in a way that is reasonably predictable, while also remaining compatible
with other databases, Alembic provides the **batch** operations context.

Within this context, a relational table is named, and then a series of
mutation operations to that table alone are specified within
the block.  When the context is complete, a process begins whereby the
"move and copy" procedure begins; the existing table structure is reflected
from the database, a new version of this table is created with the given
changes, data is copied from the
old table to the new table using "INSERT from SELECT", and finally the old
table is dropped and the new one renamed to the original name.

The :meth:`.Operations.batch_alter_table` method provides the gateway to this
process::

    with op.batch_alter_table("some_table") as batch_op:
        batch_op.add_column(Column('foo', Integer))
        batch_op.drop_column('bar')

When the above directives are invoked within a migration script, on a
SQLite backend we would see SQL like:

.. sourcecode:: sql

    CREATE TABLE _alembic_batch_temp (
      id INTEGER NOT NULL,
      foo INTEGER,
      PRIMARY KEY (id)
    );
    INSERT INTO _alembic_batch_temp (id) SELECT some_table.id FROM some_table;
    DROP TABLE some_table;
    ALTER TABLE _alembic_batch_temp RENAME TO some_table;

On other backends, we'd see the usual ``ALTER`` statements done as though
there were no batch directive - the batch context by default only does
the "move and copy" process if SQLite is in use, and if there are
migration directives other than :meth:`.Operations.add_column` present,
which is the one kind of column-level ALTER statement that SQLite supports.
:meth:`.Operations.batch_alter_table` can be configured
to run "move and copy" unconditionally in all cases, including on databases
other than SQLite; more on this is below.


Dealing with Constraints
------------------------

One area of difficulty with "move and copy" is that of constraints.  If
the SQLite database is enforcing referential integrity with
``PRAGMA FOREIGN KEYS``, this pragma may need to be disabled when the workflow
mode proceeds, else remote constraints which refer to this table may prevent
it from being dropped; additionally, for referential integrity to be
re-enabled, it may be necessary to recreate the
foreign keys on those remote tables to refer again to the new table (this
is definitely the case on other databases, at least).  SQLite is normally used
without referential integrity enabled so this won't be a problem for most
users.

"Move and copy" also currently does not account for CHECK constraints, assuming
table reflection is used.   If the table being recreated has any CHECK
constraints, they need to be specified explicitly, such as using
:paramref:`.Operations.batch_alter_table.table_args`::

    with op.batch_alter_table("some_table", table_args=[
          CheckConstraint('x > 5')
      ]) as batch_op:
        batch_op.add_column(Column('foo', Integer))
        batch_op.drop_column('bar')

For UNIQUE constraints, SQLite unlike any other database supports the concept
of a UNIQUE constraint that has no name at all; all other backends always
assign a name of some kind to all constraints that are otherwise not named
when they are created.   In SQLAlchemy, an unnamed UNIQUE constraint is
implicit when the ``unique=True`` flag is present on a
:class:`~sqlalchemy.schema.Column`, so on SQLite these constraints will
remain unnamed.

The issue here is that SQLAlchemy until version 1.0 does not report on these
SQLite-only unnamed constraints when the table is reflected.   So to support
the recreation of unnamed UNIQUE constraints, either they should be named
in the first place, or again specified within
:paramref:`.Operations.batch_alter_table.table_args`.

Working in Offline Mode
-----------------------

Another big limitation of "move and copy" is that in order to make a copy
of a table, the structure of that table must be known.
:meth:`.Operations.batch_alter_table` by default will use reflection to
get this information, which means that "online" mode is required; the
``--sql`` flag **cannot** be used without extra steps.

To support offline mode, the system must work without table reflection
present, which means the full table as it intends to be created must be
passed to :meth:`.Operations.batch_alter_table` using
:paramref:`.Operations.batch_alter_table.copy_from`::

    meta = MetaData()
    some_table = Table(
        'some_table', meta,
        Column('id', Integer, primary_key=True),
        Column('bar', String(50))
    )

    with op.batch_alter_table("some_table", copy_from=some_table) as batch_op:
        batch_op.add_column(Column('foo', Integer))
        batch_op.drop_column('bar')

The above use pattern is pretty tedious and quite far off from Alembic's
preferred style of working; however, if one needs to do SQLite-compatible
"move and copy" migrations and need them to generate flat SQL files in
"offline" mode, there's not much alternative.


Batch mode with Autogenerate
----------------------------

The syntax of batch mode is essentially that :meth:`.Operations.batch_alter_table`
is used to enter a batch block, and the returned :class:`.BatchOperations` context
works just like the regular :class:`.Operations` context, except that
the "table name" and "schema name" arguments are omitted.

To support rendering of migration commands in batch mode for autogenerate,
configure the :paramref:`.EnvironmentContext.configure.render_as_batch`
flag in ``env.py``::

    context.configure(
        connection=connection,
        target_metadata=target_metadata,
        render_as_batch=True
    )

Autogenerate will now generate along the lines of::

    def upgrade():
        ### commands auto generated by Alembic - please adjust! ###
        with op.batch_alter_table('address', schema=None) as batch_op:
            batch_op.add_column(sa.Column('street', sa.String(length=50), nullable=True))

This mode is safe to use in all cases, as the :meth:`.Operations.batch_alter_table`
directive by default only takes place for SQLite; other backends will
behave just as they normally do in the absense of the batch directives.

Note that autogenerate support does not include "offline" mode, where
the :paramref:`.Operations.batch_alter_table.copy_from` parameter is used.
The table definition here would need to be entered into migration files
manually if this is needed.

Batch mode with databases other than SQLite
--------------------------------------------

There's an odd use case some shops have, where the "move and copy" style
of migration is useful in some cases for databases that do already support
ALTER.   There's some cases where an ALTER operation may block access to the
table for a long time, which might not be acceptable.  "move and copy" can
be made to work on other backends, though with a few extra caveats.

The batch mode directive will run the "recreate" system regardless of
backend if the flag ``recreate='always'`` is passed::

    with op.batch_alter_table("some_table", recreate='always') as batch_op:
        batch_op.add_column(Column('foo', Integer))

The issues that arise in this mode are mostly to do with constraints.
Databases such as Postgresql and MySQL with InnoDB will enforce referential
integrity (e.g. via foreign keys) in all cases.   Unlike SQLite, it's not
as simple to turn off referential integrity across the board (nor would it
be desirable).    Since a new table is replacing the old one, existing
foreign key constraints which refer to the target table will need to be
unconditionally dropped before the batch operation, and re-created to refer
to the new table afterwards.  Batch mode currently does not provide any
automation for this.

The Postgresql database and possibly others also have the behavior such
that when the new table is created, a naming conflict occurs with the
named constraints of the new table, in that they match those of the old
table, and on Postgresql, these names need to be unique across all tables.
The Postgresql dialect will therefore emit a "DROP CONSTRAINT" directive
for all constraints on the old table before the new one is created; this is
"safe" in case of a failed operation because Postgresql also supports
transactional DDL.

Note that also as is the case with SQLite, CHECK constraints need to be
moved over between old and new table manually using the
:paramref:`.Operations.batch_alter_table.table_args` parameter.


.. _tutorial_constraint_names:

The Importance of Naming Constraints
====================================

An important topic worth mentioning is that of constraint naming conventions.
As we've proceeded here, we've talked about adding tables and columns, and
we've also hinted at lots of other operations listed in :ref:`ops` such as those
which support adding or dropping constraints like foreign keys and unique
constraints.   The way these constraints are referred to in migration scripts
is by name, however these names by default are in most cases generated by
the relational database in use, when the constraint is created.  For example,
if you emitted two CREATE TABLE statements like this on Postgresql::

  test=> CREATE TABLE user_account (id INTEGER PRIMARY KEY);
  CREATE TABLE
  test=> CREATE TABLE user_order (
  test(>   id INTEGER PRIMARY KEY,
  test(>   user_account_id INTEGER REFERENCES user_account(id));
  CREATE TABLE

Suppose we wanted to DROP the REFERENCES that we just applied to the
``user_order.user_account_id`` column, how do we do that?  At the prompt,
we'd use ``ALTER TABLE <tablename> DROP CONSTRAINT <constraint_name>``, or if
using Alembic we'd be using :meth:`.Operations.drop_constraint`.  But both
of those functions need a name - what's the name of this constraint?

It does have a name, which in this case we can figure out by looking at the
Postgresql catalog tables::

  test=> SELECT r.conname FROM
  test->  pg_catalog.pg_class c JOIN pg_catalog.pg_namespace n ON n.oid = c.relnamespace
  test->  JOIN pg_catalog.pg_constraint r  ON c.oid = r.conrelid
  test->  WHERE c.relname='user_order' AND r.contype = 'f'
  test-> ;
               conname
  ---------------------------------
   user_order_user_account_id_fkey
  (1 row)

The name above is not something that Alembic or SQLAlchemy created;
``user_order_user_account_id_fkey`` is a naming scheme used internally by
Postgresql to name constraints that are otherwise not named.

This scheme doesn't seem so complicated, and we might want to just use our
knowledge of it so that we know what name to use for our
:meth:`.Operations.drop_constraint` call.  But is that a good idea?   What
if for example we needed our code to run on Oracle as well.  OK, certainly
Oracle uses this same scheme, right?  Or if not, something similar.  Let's
check::

  Oracle Database 10g Express Edition Release 10.2.0.1.0 - Production

  SQL> CREATE TABLE user_account (id INTEGER PRIMARY KEY);

  Table created.

  SQL> CREATE TABLE user_order (
    2     id INTEGER PRIMARY KEY,
    3     user_account_id INTEGER REFERENCES user_account(id));

  Table created.

  SQL> SELECT constraint_name FROM all_constraints WHERE
    2     table_name='USER_ORDER' AND constraint_type in ('R');

  CONSTRAINT_NAME
  -----------------------------------------------------
  SYS_C0029334

Oh, we can see that is.....much worse.  Oracle's names are entirely unpredictable
alphanumeric codes, and this will make being able to write migrations
quite tedious, as we'd need to look up all these names.

The solution to having to look up names is to make your own names.   This is
an easy, though tedious thing to do manually.  For example, to create our model
in SQLAlchemy ensuring we use names for foreign key constraints would look like::

  from sqlalchemy import MetaData, Table, Column, Integer, ForeignKey

  meta = MetaData()

  user_account = Table('user_account', meta,
                    Column('id', Integer, primary_key=True)
                )

  user_order = Table('user_order', meta,
                    Column('id', Integer, primary_key=True),
                    Column('user_order_id', Integer,
                      ForeignKey('user_account.id', name='fk_user_order_id'))
                )

Simple enough, though this has some disadvantages.  The first is that it's tedious;
we need to remember to use a name for every :class:`~sqlalchemy.schema.ForeignKey` object,
not to mention every :class:`~sqlalchemy.schema.UniqueConstraint`, :class:`~sqlalchemy.schema.CheckConstraint`,
:class:`~sqlalchemy.schema.Index`, and maybe even :class:`~sqlalchemy.schema.PrimaryKeyConstraint`
as well if we wish to be able to alter those too, and beyond all that, all the
names have to be globally unique.   Even with all that effort, if we have a naming scheme in mind,
it's easy to get it wrong when doing it manually each time.

What's worse is that manually naming constraints (and indexes) gets even more
tedious in that we can no longer use convenience features such as the ``.unique=True``
or ``.index=True`` flag on :class:`~sqlalchemy.schema.Column`::

  user_account = Table('user_account', meta,
                    Column('id', Integer, primary_key=True),
                    Column('name', String(50), unique=True)
                )

Above, the ``unique=True`` flag creates a :class:`~sqlalchemy.schema.UniqueConstraint`, but again,
it's not named.   If we want to name it, manually we have to forego the usage
of ``unique=True`` and type out the whole constraint::

  user_account = Table('user_account', meta,
                    Column('id', Integer, primary_key=True),
                    Column('name', String(50)),
                    UniqueConstraint('name', name='uq_user_account_name')
                )

There's a solution to all this naming work, which is to use an **automated
naming convention**.  For some years, SQLAlchemy has encourgaged the use of
DDL Events in order to create naming schemes.  The :meth:`~sqlalchemy.events.DDLEvents.after_parent_attach`
event in particular is the best place to intercept when :class:`~sqlalchemy.schema.Constraint`
and :class:`~sqlalchemy.schema.Index` objects are being associated with a parent
:class:`~sqlalchemy.schema.Table` object, and to assign a ``.name`` to the constraint while making
use of the name of the table and associated columns.

But there is also a better way to go, which is to make use of a feature
new in SQLAlchemy 0.9.2 which makes use of the events behind the scenes known as
:paramref:`~sqlalchemy.schema.MetaData.naming_convention`.   Here, we can
create a new :class:`~sqlalchemy.schema.MetaData` object while passing a dictionary referring
to a naming scheme::

    convention = {
      "ix": 'ix_%(column_0_label)s',
      "uq": "uq_%(table_name)s_%(column_0_name)s",
      "ck": "ck_%(table_name)s_%(constraint_name)s",
      "fk": "fk_%(table_name)s_%(column_0_name)s_%(referred_table_name)s",
      "pk": "pk_%(table_name)s"
    }

    metadata = MetaData(naming_convention=convention)

If we define our models using a :class:`~sqlalchemy.schema.MetaData` as above, the given
naming convention dictionary will be used to provide names for all constraints
and indexes.

.. _autogen_naming_conventions:

Integration of Naming Conventions into Operations, Autogenerate
---------------------------------------------------------------

As of Alembic 0.6.4, the naming convention feature is integrated into the
:class:`.Operations` object, so that the convention takes effect for any
constraint that is otherwise unnamed.  The naming convention is passed to
:class:`.Operations` using the :paramref:`.MigrationsContext.configure.target_metadata`
parameter in ``env.py``, which is normally configured when autogenerate is
used::

    # in your application's model:

    meta = MetaData(naming_convention={
            "ix": 'ix_%(column_0_label)s',
            "uq": "uq_%(table_name)s_%(column_0_name)s",
            "ck": "ck_%(table_name)s_%(constraint_name)s",
            "fk": "fk_%(table_name)s_%(column_0_name)s_%(referred_table_name)s",
            "pk": "pk_%(table_name)s"
          })

    # .. in your Alembic env.py:

    # add your model's MetaData object here
    # for 'autogenerate' support
    from myapp import mymodel
    target_metadata = mymodel.Base.metadata

    # ...

    def run_migrations_online():

        # ...

        context.configure(
                    connection=connection,
                    target_metadata=target_metadata
                    )

Above, when we render a directive like the following::

    op.add_column('sometable', Column('q', Boolean(name='q_bool')))

The Boolean type will render a CHECK constraint with the name
``"ck_sometable_q_bool"``, assuming the backend in use does not support
native boolean types.

We can also use op directives with constraints and not give them a name
at all, if the naming convention doesn't require one.  The value of
``None`` will be converted into a name that follows the appopriate naming
conventions::

    def upgrade():
        op.create_unique_constraint(None, 'some_table', 'x')

When autogenerate renders constraints in a migration script, it renders them
typically with their completed name.  If using at least Alembic 0.6.4 as well
as SQLAlchemy 0.9.4, these will be rendered with a special directive
:meth:`.Operations.f` which denotes that the string has already been
tokenized::

    def upgrade():
        op.create_unique_constraint(op.f('uq_const_x'), 'some_table', 'x')


For more detail on the naming convention feature, see :ref:`sqla:constraint_naming_conventions`.


.. _branches:

Working with Branches
=====================

.. note:: Alembic 0.7.0 features an all-new versioning model that fully
   supports branch points, merge points, and long-lived, labeled branches,
   including independent branches originating from multiple bases.
   A great emphasis has been placed on there being almost no impact on the
   existing Alembic workflow, including that all commands work pretty much
   the same as they did before, the format of migration files doesn't require
   any change (though there are some changes that are recommended),
   and even the structure of the ``alembic_version``
   table does not change at all.  However, most alembic commands now offer
   new features which will break out an Alembic environment into
   "branch mode", where things become a lot more intricate.   Working in
   "branch mode" should be considered as a "beta" feature, with many new
   paradigms and use cases still to be stress tested in the wild.
   Please tread lightly!

.. versionadded:: 0.7.0

A **branch** describes a point in a migration stream when two or more
versions refer to the same parent migration as their anscestor.  Branches
occur naturally when two divergent source trees, both containing Alembic
revision files created independently within those source trees, are merged
together into one.  When this occurs, the challenge of a branch is to **merge** the
branches into a single series of changes, so that databases established
from either source tree individually can be upgraded to reference the merged
result equally.  Another scenario where branches are present are when we create them
directly; either at some point in the migration stream we'd like different
series of migrations to be managed independently (e.g. we create a tree),
or we'd like separate migration streams for different features starting
at the root (e.g. a *forest*).  We'll illustrate all of these cases, starting
with the most common which is a source-merge-originated branch that we'll
merge.

Starting with the "account table" example we began in :ref:`create_migration`,
assume we have our basemost version ``1975ea83b712``, which leads into
the second revision ``ae1027a6acf``, and the migration files for these
two revisions are checked into our source repository.
Consider if we merged into our source repository another code branch which contained
a revision for another table called ``shopping_cart``.  This revision was made
against our first Alembic revision, the one that generated ``account``.   After
loading the second source tree in, a new file
``27c6a30d7c24_add_shopping_cart_table.py`` exists within our ``versions`` directory.
Both it, as well as ``ae1027a6acf_add_a_column.py``, reference
``1975ea83b712_add_account_table.py`` as the "downgrade" revision.  To illustrate::

    # main source tree:
    1975ea83b712 (add account table) -> ae1027a6acf (add a column)

    # branched source tree
    1975ea83b712 (add account table) -> 27c6a30d7c24 (add shopping cart table)

Above, we can see ``1975ea83b712`` is our **branch point**; two distinct versions
both refer to it as its parent.  The Alembic command ``branches`` illustrates
this fact::

  $ alembic branches --verbose
  Rev: 1975ea83b712 (branchpoint)
  Parent: <base>
  Branches into: 27c6a30d7c24, ae1027a6acf
  Path: foo/versions/1975ea83b712_add_account_table.py

      add account table

      Revision ID: 1975ea83b712
      Revises:
      Create Date: 2014-11-20 13:02:46.257104

               -> 27c6a30d7c24 (head), add shopping cart table
               -> ae1027a6acf (head), add a column

History shows it too, illustrating two ``head`` entries as well
as a ``branchpoint``::

    $ alembic history
    1975ea83b712 -> 27c6a30d7c24 (head), add shopping cart table
    1975ea83b712 -> ae1027a6acf (head), add a column
    <base> -> 1975ea83b712 (branchpoint), add account table

We can get a view of just the current heads using ``alembic heads``::

    $ alembic heads --verbose
    Rev: 27c6a30d7c24 (head)
    Parent: 1975ea83b712
    Path: foo/versions/27c6a30d7c24_add_shopping_cart_table.py

        add shopping cart table

        Revision ID: 27c6a30d7c24
        Revises: 1975ea83b712
        Create Date: 2014-11-20 13:03:11.436407

    Rev: ae1027a6acf (head)
    Parent: 1975ea83b712
    Path: foo/versions/ae1027a6acf_add_a_column.py

        add a column

        Revision ID: ae1027a6acf
        Revises: 1975ea83b712
        Create Date: 2014-11-20 13:02:54.849677

If we try to run an ``upgrade`` to the usual end target of ``head``, Alembic no
longer considers this to be an unambiguous command.  As we have more than
one ``head``, the ``upgrade`` command wants us to provide more information::

    $ alembic upgrade head
      FAILED: Multiple head revisions are present for given argument 'head'; please specify a specific
      target revision, '<branchname>@head' to narrow to a specific head, or 'heads' for all heads

The ``upgrade`` command gives us quite a few options in which we can proceed
with our upgrade, either giving it information on *which* head we'd like to upgrade
towards, or alternatively stating that we'd like *all* heads to be upgraded
towards at once.  However, in the typical case of two source trees being
merged, we will want to pursue a third option, which is that we can **merge** these
branches.

Merging Branches
----------------

An Alembic merge is a migration file that joins two or
more "head" files together. If the two branches we have right now can
be said to be a "tree" structure, introducing this merge file will
turn it into a "diamond" structure::

                                -- ae1027a6acf -->
                               /                   \
    <base> --> 1975ea83b712 -->                      --> mergepoint
                               \                   /
                                -- 27c6a30d7c24 -->

We create the merge file using ``alembic merge``; with this command, we can
pass to it an argument such as ``heads``, meaning we'd like to merge all
heads.  Or, we can pass it individual revision numbers sequentally::

    $ alembic merge -m "merge ae1 and 27c" ae1027 27c6a
      Generating /path/to/foo/versions/53fffde5ad5_merge_ae1_and_27c.py ... done

Looking inside the new file, we see it as a regular migration file, with
the only new twist is that ``down_revision`` points to both revisions::

    """merge ae1 and 27c

    Revision ID: 53fffde5ad5
    Revises: ae1027a6acf, 27c6a30d7c24
    Create Date: 2014-11-20 13:31:50.811663

    """

    # revision identifiers, used by Alembic.
    revision = '53fffde5ad5'
    down_revision = ('ae1027a6acf', '27c6a30d7c24')
    branch_labels = None

    from alembic import op
    import sqlalchemy as sa


    def upgrade():
        pass


    def downgrade():
        pass

This file is a regular migration file, and if we wish to, we may place
:class:`.Operations` directives into the ``upgrade()`` and ``downgrade()``
functions like any other migration file.  Though it is probably best to limit
the instructions placed here only to those that deal with any kind of
reconciliation that is needed between the two merged branches, if any.

The ``heads`` command now illustrates that the multiple heads in our
``versions/`` directory have been resolved into our new head::

    $ alembic heads --verbose
    Rev: 53fffde5ad5 (head) (mergepoint)
    Merges: ae1027a6acf, 27c6a30d7c24
    Path: foo/versions/53fffde5ad5_merge_ae1_and_27c.py

        merge ae1 and 27c

        Revision ID: 53fffde5ad5
        Revises: ae1027a6acf, 27c6a30d7c24
        Create Date: 2014-11-20 13:31:50.811663

History shows a similar result, as the mergepoint becomes our head::

    $ alembic history
    ae1027a6acf, 27c6a30d7c24 -> 53fffde5ad5 (head) (mergepoint), merge ae1 and 27c
    1975ea83b712 -> ae1027a6acf, add a column
    1975ea83b712 -> 27c6a30d7c24, add shopping cart table
    <base> -> 1975ea83b712 (branchpoint), add account table

With a single ``head`` target, a generic ``upgrade`` can proceed::

    $ alembic upgrade head
    INFO  [alembic.migration] Context impl PostgresqlImpl.
    INFO  [alembic.migration] Will assume transactional DDL.
    INFO  [alembic.migration] Running upgrade  -> 1975ea83b712, add account table
    INFO  [alembic.migration] Running upgrade 1975ea83b712 -> 27c6a30d7c24, add shopping cart table
    INFO  [alembic.migration] Running upgrade 1975ea83b712 -> ae1027a6acf, add a column
    INFO  [alembic.migration] Running upgrade ae1027a6acf, 27c6a30d7c24 -> 53fffde5ad5, merge ae1 and 27c


.. topic:: merge mechanics

  The upgrade process traverses through all of our migration files using
  a  **topological sorting** algorithm, treating the list of migration
  files not as a linked list, but as a **directed acyclic graph**.  The starting
  points of this traversal are the **current heads** within our database,
  and the end point is the "head" revision or revisions specified.

  When a migration proceeds across a point at which there are multiple heads,
  the ``alembic_version`` table will at that point store *multiple* rows,
  one for each head.  Our migration process above will emit SQL against
  ``alembic_version`` along these lines:

    .. sourcecode:: sql

      -- Running upgrade  -> 1975ea83b712, add account table
      INSERT INTO alembic_version (version_num) VALUES ('1975ea83b712')

      -- Running upgrade 1975ea83b712 -> 27c6a30d7c24, add shopping cart table
      UPDATE alembic_version SET version_num='27c6a30d7c24' WHERE alembic_version.version_num = '1975ea83b712'

      -- Running upgrade 1975ea83b712 -> ae1027a6acf, add a column
      INSERT INTO alembic_version (version_num) VALUES ('ae1027a6acf')

      -- Running upgrade ae1027a6acf, 27c6a30d7c24 -> 53fffde5ad5, merge ae1 and 27c
      DELETE FROM alembic_version WHERE alembic_version.version_num = 'ae1027a6acf'
      UPDATE alembic_version SET version_num='53fffde5ad5' WHERE alembic_version.version_num = '27c6a30d7c24'

  At the point at which both ``27c6a30d7c24`` and ``ae1027a6acf`` exist within our
  database, both values are present in ``alembic_version``, which now has
  two rows.   If we upgrade to these two versions alone, then stop and
  run ``alembic current``, we will see this::

      $ alembic current --verbose
      Current revision(s) for postgresql://scott:XXXXX@localhost/test:
      Rev: ae1027a6acf
      Parent: 1975ea83b712
      Path: foo/versions/ae1027a6acf_add_a_column.py

          add a column

          Revision ID: ae1027a6acf
          Revises: 1975ea83b712
          Create Date: 2014-11-20 13:02:54.849677

      Rev: 27c6a30d7c24
      Parent: 1975ea83b712
      Path: foo/versions/27c6a30d7c24_add_shopping_cart_table.py

          add shopping cart table

          Revision ID: 27c6a30d7c24
          Revises: 1975ea83b712
          Create Date: 2014-11-20 13:03:11.436407

  A key advantage to the ``merge`` process is that it will
  run equally well on databases that were present on version ``ae1027a6acf``
  alone, versus databases that were present on version ``27c6a30d7c24`` alone;
  whichever version was not yet applied, will be applied before the merge point
  can be crossed.   This brings forth a way of thinking about a merge file,
  as well as about any Alembic revision file.  As they are considered to
  be "nodes" within a set that is subject to topological sorting, each
  "node" is a point that cannot be crossed until all of its dependencies
  are satisfied.

  Prior to Alembic's support of merge points, the use case of databases
  sitting on different heads was basically impossible to reconcile; having
  to manually splice the head files together invariably meant that one migration
  would occur before the other, thus being incompatible with databases that
  were present on the other migration.

Working with Explicit Branches
------------------------------

The ``alembic upgrade`` command hinted at other options besides merging when
dealing with multiple heads.  Let's back up and assume we're back where
we have as our heads just ``ae1027a6acf`` and ``27c6a30d7c24``::

    $ alembic heads
    27c6a30d7c24
    ae1027a6acf

Earlier, when we did ``alembic upgrade head``, it gave us an error which
suggested ``please specify a specific target revision, '<branchname>@head' to
narrow to a specific head, or 'heads' for all heads`` in order to proceed
without merging.   Let's cover those cases.

Referring to all heads at once
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

The ``heads`` identifier is a lot like ``head``, except it explicitly refers
to *all* heads at once.  That is, it's like telling Alembic to do the operation
for both ``ae1027a6acf`` and ``27c6a30d7c24`` simultaneously.  If we started
from a fresh database and ran ``upgrade heads`` we'd see::

    $ alembic upgrade heads
    INFO  [alembic.migration] Context impl PostgresqlImpl.
    INFO  [alembic.migration] Will assume transactional DDL.
    INFO  [alembic.migration] Running upgrade  -> 1975ea83b712, add account table
    INFO  [alembic.migration] Running upgrade 1975ea83b712 -> ae1027a6acf, add a column
    INFO  [alembic.migration] Running upgrade 1975ea83b712 -> 27c6a30d7c24, add shopping cart table

Since we've upgraded to ``heads``, and we do in fact have more than one head,
that means these two distinct heads are now in our ``alembic_version`` table.
We can see this if we run ``alembic current``::

    $ alembic current
    ae1027a6acf (head)
    27c6a30d7c24 (head)

That means there's two rows in ``alembic_version`` right now.  If we downgrade
one step at a time, Alembic will **delete** from the ``alembic_version`` table
each branch that's closed out, until only one branch remains; then it will
continue updating the single value down to the previous versions::

    $ alembic downgrade -1
    INFO  [alembic.migration] Running downgrade ae1027a6acf -> 1975ea83b712, add a column

    $ alembic current
    27c6a30d7c24 (head)

    $ alembic downgrade -1
    INFO  [alembic.migration] Running downgrade 27c6a30d7c24 -> 1975ea83b712, add shopping cart table

    $ alembic current
    1975ea83b712 (branchpoint)

    $ alembic downgrade -1
    INFO  [alembic.migration] Running downgrade 1975ea83b712 -> , add account table

    $ alembic current

Referring to a Specific Version
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

We can pass a specific version number to ``upgrade``.  Alembic will ensure that
all revisions upon which this version depends are invoked, and nothing more.
So if we ``upgrade`` either to ``27c6a30d7c24`` or ``ae1027a6acf`` specifically,
it guarantees that ``1975ea83b712`` will have been applied, but not that
any "sibling" versions are applied::

    $ alembic upgrade 27c6a
    INFO  [alembic.migration] Running upgrade  -> 1975ea83b712, add account table
    INFO  [alembic.migration] Running upgrade 1975ea83b712 -> 27c6a30d7c24, add shopping cart table

With ``1975ea83b712`` and ``27c6a30d7c24`` applied, ``ae1027a6acf`` is just
a single additional step::

    $ alembic upgrade ae102
    INFO  [alembic.migration] Running upgrade 1975ea83b712 -> ae1027a6acf, add a column

Working with Branch Labels
^^^^^^^^^^^^^^^^^^^^^^^^^^

To satisfy the use case where an environment has long-lived branches, especially
independent branches as will be discussed in the next section, Alembic supports
the concept of **branch labels**.   These are string values that are present
within the migration file, using the new identifier ``branch_labels``.
For example, if we want to refer to the "shopping cart" branch using the name
"shoppingcart", we can add that name to our file
``27c6a30d7c24_add_shopping_cart_table.py``::

    """add shopping cart table

    """

    # revision identifiers, used by Alembic.
    revision = '27c6a30d7c24'
    down_revision = '1975ea83b712'
    branch_labels = ('shoppingcart',)

    # ...

The ``branch_labels`` attribute refers to a string name, or a tuple
of names, which will now apply to this revision, all descendants of this
revision, as well as all ancestors of this revision up until the preceding
branch point, in this case ``1975ea83b712``.  We can see the ``shoppingcart``
label applied to this revision::

    $ alembic history
    1975ea83b712 -> 27c6a30d7c24 (shoppingcart) (head), add shopping cart table
    1975ea83b712 -> ae1027a6acf (head), add a column
    <base> -> 1975ea83b712 (branchpoint), add account table

With the label applied, the name ``shoppingcart`` now serves as an alias
for the ``27c6a30d7c24`` revision specifically.  We can illustrate this
by showing it with ``alembic show``::

    $ alembic show shoppingcart
    Rev: 27c6a30d7c24 (head)
    Parent: 1975ea83b712
    Branch names: shoppingcart
    Path: foo/versions/27c6a30d7c24_add_shopping_cart_table.py

        add shopping cart table

        Revision ID: 27c6a30d7c24
        Revises: 1975ea83b712
        Create Date: 2014-11-20 13:03:11.436407

However, when using branch labels, we usually want to use them using a syntax
known as "branch at" syntax; this syntax allows us to state that we want to
use a specific revision, let's say a "head" revision, in terms of a *specific*
branch.  While normally, we can't refer to ``alembic upgrade head`` when
there's multiple heads, we *can* refer to this head specifcally using
``shoppingcart@head`` syntax::

    $ alembic upgrade shoppingcart@head
    INFO  [alembic.migration] Running upgrade 1975ea83b712 -> 27c6a30d7c24, add shopping cart table

The ``shoppingcart@head`` syntax becomes important to us if we wish to
add new migration files to our versions directory while maintaining multiple
branches.  Just like the ``upgrade`` command, if we attempted to add a new
revision file to our multiple-heads layout without a specific parent revision,
we'd get a familiar error::

    $ alembic revision -m "add a shopping cart column"
      FAILED: Multiple heads are present; please specify the head revision on
      which the new revision should be based, or perform a merge.

The ``alembic revision`` command is pretty clear in what we need to do;
to add our new revision specifically to the ``shoppingcart`` branch,
we use the ``--head`` argument, either with the specific revision identifier
``27c6a30d7c24``, or more generically using our branchname ``shoppingcart@head``::

    $ alembic revision -m "add a shopping cart column"  --head shoppingcart@head
      Generating /path/to/foo/versions/d747a8a8879_add_a_shopping_cart_column.py ... done

``alembic history`` shows both files now part of the ``shoppingcart`` branch::

    $ alembic history
    1975ea83b712 -> ae1027a6acf (head), add a column
    27c6a30d7c24 -> d747a8a8879 (shoppingcart) (head), add a shopping cart column
    1975ea83b712 -> 27c6a30d7c24 (shoppingcart), add shopping cart table
    <base> -> 1975ea83b712 (branchpoint), add account table

We can limit our history operation just to this branch as well::

    $ alembic history -r shoppingcart:
    27c6a30d7c24 -> d747a8a8879 (shoppingcart) (head), add a shopping cart column
    1975ea83b712 -> 27c6a30d7c24 (shoppingcart), add shopping cart table

If we want to illustrate the path of ``shoppingcart`` all the way from the
base, we can do that as follows::

    $ alembic history -r :shoppingcart@head
    27c6a30d7c24 -> d747a8a8879 (shoppingcart) (head), add a shopping cart column
    1975ea83b712 -> 27c6a30d7c24 (shoppingcart), add shopping cart table
    <base> -> 1975ea83b712 (branchpoint), add account table

We can run this operation from the "base" side as well, but we get a different
result::

    $ alembic history -r shoppingcart@base:
    1975ea83b712 -> ae1027a6acf (head), add a column
    27c6a30d7c24 -> d747a8a8879 (shoppingcart) (head), add a shopping cart column
    1975ea83b712 -> 27c6a30d7c24 (shoppingcart), add shopping cart table
    <base> -> 1975ea83b712 (branchpoint), add account table

When we list from ``shoppingcart@base`` without an endpoint, it's really shorthand
for ``-r shoppingcart@base:heads``, e.g. all heads, and since ``shoppingcart@base``
is the same "base" shared by the ``ae1027a6acf`` revision, we get that
revision in our listing as well.  The ``<branchname>@base`` syntax can be
useful when we are dealing with individual bases, as we'll see in the next
section.

The ``<branchname>@head`` format can also be used with revision numbers
instead of branch names, though this is less convenient.  If we wanted to
add a new revision to our branch that includes the un-labeled ``ae1027a6acf``,
if this weren't a head already, we could ask for the "head of the branch
that includes ``ae1027a6acf``" as follows::

    $ alembic revision -m "add another account column" --head ae10@head
      Generating /Users/classic/dev/alembic/foo/versions/55af2cb1c267_add_another_account_column.py ... done

More Label Syntaxes
^^^^^^^^^^^^^^^^^^^

The ``heads`` symbol can be combined with a branch label, in the case that
your labeled branch itself breaks off into multiple branches::

    $ alembic upgrade shoppingcart@heads

Relative identifiers, as introduced in :ref:`relative_migrations`,
work with labels too.  For example, upgrading to ``shoppingcart@+2``
means to upgrade from current heads on "shoppingcart" upwards two revisions::

    $ alembic upgrade shoppingcart@+2

This kind of thing works from history as well::

    $ alembic history -r current:shoppingcart@+2


Working with Multiple Bases
---------------------------

We've seen in the previous section that ``alembic upgrade`` is fine
if we have multiple heads, ``alembic revision`` allows us to tell it which
"head" we'd like to associate our new revision file with, and branch labels
allow us to assign names to branches that we can use in subsequent commands.
Let's put all these together and refer to a new "base", that is, a whole
new tree of revision files that will be semi-independent of the account/shopping
cart revisions we've been working with.

Creating a Labeled Base Revision
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

We want to create a new, labeled branch in one step.  To ensure the branch can
accommodate this label, we need to ensure our ``script.py.mako`` file, used
for generating new revision files, has the appropriate substitutions present.
If Alembic version 0.7.0 or greater was used to generate the original
migration environment, this is already done.  However when working with an older
environment, ``script.py.mako`` needs to have this directive added, typically
underneath the ``down_revision`` directive::

    # revision identifiers, used by Alembic.
    revision = ${repr(up_revision)}
    down_revision = ${repr(down_revision)}

    # add this here in order to use revision with branch_label
    branch_labels = ${repr(branch_labels)}

With this in place, we can create a new revision file, starting up a branch
that will deal with database tables involving networking; we specify the
"head" version of ``base`` as well as a ``branch_label``::

    $ alembic revision -m "create networking branch" --head=base --branch-label=networking
      Generating /Users/classic/dev/alembic/foo/versions/3782d9986ced_create_networking_branch.py ... done

If we ran the above command and we didn't have the newer ``script.py.mako``
directive, we'd get this error::

  FAILED: Version 3cac04ae8714 specified branch_labels networking, however
  the migration file foo/versions/3cac04ae8714_create_networking_branch.py
  does not have them; have you upgraded your script.py.mako to include the 'branch_labels'
  section?

When we receive the above error, and we would like to try again, we need to
either **delete** the incorrectly generated file in order to run ``revision``
again, *or* we can edit the ``3cac04ae8714_create_networking_branch.py``
directly to add the ``branch_labels`` in of our choosing.

Running with Multiple Bases
^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Once we have a new, permanent (for as long as we desire it to be)
base in our system, we'll always have multiple heads present::

    $ alembic heads
    3782d9986ced (networking)
    ae1027a6acf
    d747a8a8879 (shoppingcart)

When we want to add a new revision file to ``networking``, we specify
``networking@head`` as the ``--head``::

    $ alembic revision -m "add ip number table" --head=networking@head
      Generating /Users/classic/dev/alembic/foo/versions/109ec7d132bf_add_ip_number_table.py ... done

It's important that we refer to the head using ``networking@head``; if we
only refer to ``networking``, that refers to only ``3782d9986ced`` specifically;
if we specify this and it's not a head, ``alembic revision`` will make sure
we didn't mean to specify the head::

    $ alembic revision -m "add DNS table" --head=networking
      FAILED: Revision 3782d9986ced is not a head revision; please
      specify --splice to create a new branch from this revision

As mentioned earlier, as this base is independent, we can view its history
from the base using ``history -r networking@base:``::

    $ alembic history -r networking@base:
    109ec7d132bf -> 29f859a13ea (networking) (head), add DNS table
    3782d9986ced -> 109ec7d132bf (networking), add ip number table
    <base> -> 3782d9986ced (networking), create networking branch

Note this is the same output we'd get at this point if we used
``-r :networking@head``.

We may now run upgrades or downgrades freely, among individual branches
(let's assume a clean database again)::

    $ alembic upgrade networking@head
    INFO  [alembic.migration] Running upgrade  -> 3782d9986ced, create networking branch
    INFO  [alembic.migration] Running upgrade 3782d9986ced -> 109ec7d132bf, add ip number table
    INFO  [alembic.migration] Running upgrade 109ec7d132bf -> 29f859a13ea, add DNS table

or against the whole thing using ``heads``::

    $ alembic upgrade heads
    INFO  [alembic.migration] Running upgrade  -> 1975ea83b712, add account table
    INFO  [alembic.migration] Running upgrade 1975ea83b712 -> 27c6a30d7c24, add shopping cart table
    INFO  [alembic.migration] Running upgrade 27c6a30d7c24 -> d747a8a8879, add a shopping cart column
    INFO  [alembic.migration] Running upgrade 1975ea83b712 -> ae1027a6acf, add a column
    INFO  [alembic.migration] Running upgrade ae1027a6acf -> 55af2cb1c267, add another account column

Branch and Merge Nuttiness
--------------------------

We have quite a lot of versioning going on, history overall now shows::

    $ alembic history
    109ec7d132bf -> 29f859a13ea (networking) (head), add DNS table
    3782d9986ced -> 109ec7d132bf (networking), add ip number table
    <base> -> 3782d9986ced (networking), create networking branch
    ae1027a6acf -> 55af2cb1c267 (head), add another account column
    1975ea83b712 -> ae1027a6acf, add a column
    27c6a30d7c24 -> d747a8a8879 (shoppingcart) (head), add a shopping cart column
    1975ea83b712 -> 27c6a30d7c24 (shoppingcart), add shopping cart table
    <base> -> 1975ea83b712 (branchpoint), add account table


If you actually wanted, all three branches can be merged::

    $ alembic merge -m "merge all three branches" heads
      Generating /Users/classic/dev/alembic/foo/versions/3180f4d6e81d_merge_all_three_branches.py ... done

    $ alembic upgrade head
    INFO  [alembic.migration] Running upgrade 29f859a13ea, 55af2cb1c267, d747a8a8879 -> 3180f4d6e81d, merge all three branches

at which point, we're back to one head, but note!  This head has **two** labels
now::

    $ alembic heads
    3180f4d6e81d (shoppingcart, networking)

    $ alembic current --verbose
    Current revision(s) for postgresql://scott:XXXXX@localhost/test:
    Rev: 3180f4d6e81d (head) (mergepoint)
    Merges: 29f859a13ea, 55af2cb1c267, d747a8a8879
    Branch names: shoppingcart, networking
    Path: foo/versions/3180f4d6e81d_merge_all_three_branches.py

        merge all three branches

        Revision ID: 3180f4d6e81d
        Revises: 29f859a13ea, 55af2cb1c267, d747a8a8879
        Create Date: 2014-11-20 16:27:56.395477

When labels are combined like this, it means that ``networking@head`` and
``shoppingcart@head`` are ultimately along the same branch, as is the
unnamed ``ae1027a6acf`` branch since we've merged everything together.
``alembic history`` when leading from ``networking@base:``,
``:shoppingcart@head`` or similar will show the whole tree at this point::

    $ alembic history -r :shoppingcart@head
    29f859a13ea, 55af2cb1c267, d747a8a8879 -> 3180f4d6e81d (networking, shoppingcart) (head) (mergepoint), merge all three branches
    109ec7d132bf -> 29f859a13ea (networking), add DNS table
    3782d9986ced -> 109ec7d132bf (networking), add ip number table
    <base> -> 3782d9986ced (networking), create networking branch
    ae1027a6acf -> 55af2cb1c267, add another account column
    1975ea83b712 -> ae1027a6acf, add a column
    27c6a30d7c24 -> d747a8a8879 (shoppingcart), add a shopping cart column
    1975ea83b712 -> 27c6a30d7c24 (shoppingcart), add shopping cart table
    <base> -> 1975ea83b712 (branchpoint), add account table

It follows then that the "branch labels" feature is useful for branches
that are **unmerged**.  Once branches are merged into a single stream, labels
are not particularly useful as they tend to refer to the whole revision
stream in any case.  They can of course be removed from revision files
at the point at which they are no longer useful, or moved to other files.

For posterity, here's the graph of the whole thing::

                         --- ae10 --> 55af --->--
                       /                         \
    <base> --> 1975 -->                          |
                       \                         |
                         --- 27c6 --> d747 -->   |
                        (shoppingcart)        \  |
                                              +--+-----> 3180
                                                 |    (networking,
                                                /      shoppingcart)
    <base> --> 3782 -----> 109e ----> 29f8 --->
             (networking)


If there's any point to be made here, it's if you are too freely branching, merging
and labeling, things can get pretty crazy!  Hence the branching system should
be used carefully and thoughtfully for best results.


.. _building_uptodate:

Building an Up to Date Database from Scratch
=============================================

There's a theory of database migrations that says that the revisions in existence for a database should be
able to go from an entirely blank schema to the finished product, and back again.   Alembic can roll
this way.   Though we think it's kind of overkill, considering that SQLAlchemy itself can emit
the full CREATE statements for any given model using :meth:`~sqlalchemy.schema.MetaData.create_all`.   If you check out
a copy of an application, running this will give you the entire database in one shot, without the need
to run through all those migration files, which are instead tailored towards applying incremental
changes to an existing database.

Alembic can integrate with a :meth:`~sqlalchemy.schema.MetaData.create_all` script quite easily.  After running the
create operation, tell Alembic to create a new version table, and to stamp it with the most recent
revision (i.e. ``head``)::

    # inside of a "create the database" script, first create
    # tables:
    my_metadata.create_all(engine)

    # then, load the Alembic configuration and generate the
    # version table, "stamping" it with the most recent rev:
    from alembic.config import Config
    from alembic import command
    alembic_cfg = Config("/path/to/yourapp/alembic.ini")
    command.stamp(alembic_cfg, "head")

When this approach is used, the application can generate the database using normal SQLAlchemy
techniques instead of iterating through hundreds of migration scripts.   Now, the purpose of the
migration scripts is relegated just to movement between versions on out-of-date databases, not
*new* databases.    You can now remove old migration files that are no longer represented
on any existing environments.

To prune old migration files, simply delete the files.   Then, in the earliest, still-remaining
migration file, set ``down_revision`` to ``None``::

    # replace this:
    #down_revision = '290696571ad2'

    # with this:
    down_revision = None

That file now becomes the "base" of the migration series.