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========
Cookbook
========
A collection of "How-Tos", highlighting various ways to extend
Alembic.
.. note::
This is a new section where we hope to start cataloguing various "how-tos"
we come up with based on user requests. It is often the case that users
will request a feature only to learn that simple customization can
provide the same thing. There's only one recipe at the moment but
we hope to get more soon!
Conditional Migration Elements
==============================
This example features the basic idea of a common need, that of affecting
how a migration runs based on command line switches.
The technique to use here is simple; within a migration script, inspect
the :meth:`.EnvironmentContext.get_x_argument` collection for any additional,
user-defined parameters. Then take action based on the presence of those
arguments.
To make it such that the logic to inspect these flags is easy to use and
modify, we modify our ``script.py.mako`` template to make this feature
available in all new revision files:
.. code-block:: mako
"""${message}
Revision ID: ${up_revision}
Revises: ${down_revision}
Create Date: ${create_date}
"""
# revision identifiers, used by Alembic.
revision = ${repr(up_revision)}
down_revision = ${repr(down_revision)}
from alembic import op
import sqlalchemy as sa
${imports if imports else ""}
from alembic import context
def upgrade():
schema_upgrades()
if context.get_x_argument(as_dictionary=True).get('data', None):
data_upgrades()
def downgrade():
if context.get_x_argument(as_dictionary=True).get('data', None):
data_downgrades()
schema_downgrades()
def schema_upgrades():
"""schema upgrade migrations go here."""
${upgrades if upgrades else "pass"}
def schema_downgrades():
"""schema downgrade migrations go here."""
${downgrades if downgrades else "pass"}
def data_upgrades():
"""Add any optional data upgrade migrations here!"""
pass
def data_downgrades():
"""Add any optional data downgrade migrations here!"""
pass
Now, when we create a new migration file, the ``data_upgrades()`` and ``data_downgrades()``
placeholders will be available, where we can add optional data migrations::
"""rev one
Revision ID: 3ba2b522d10d
Revises: None
Create Date: 2014-03-04 18:05:36.992867
"""
# revision identifiers, used by Alembic.
revision = '3ba2b522d10d'
down_revision = None
from alembic import op
import sqlalchemy as sa
from sqlalchemy import String, Column
from sqlalchemy.sql import table, column
from alembic import context
def upgrade():
schema_upgrades()
if context.get_x_argument(as_dictionary=True).get('data', None):
data_upgrades()
def downgrade():
if context.get_x_argument(as_dictionary=True).get('data', None):
data_downgrades()
schema_downgrades()
def schema_upgrades():
"""schema upgrade migrations go here."""
op.create_table("my_table", Column('data', String))
def schema_downgrades():
"""schema downgrade migrations go here."""
op.drop_table("my_table")
def data_upgrades():
"""Add any optional data upgrade migrations here!"""
my_table = table('my_table',
column('data', String),
)
op.bulk_insert(my_table,
[
{'data': 'data 1'},
{'data': 'data 2'},
{'data': 'data 3'},
]
)
def data_downgrades():
"""Add any optional data downgrade migrations here!"""
op.execute("delete from my_table")
To invoke our migrations with data included, we use the ``-x`` flag::
alembic -x data=true upgrade head
The :meth:`.EnvironmentContext.get_x_argument` is an easy way to support
new commandline options within environment and migration scripts.
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