=============================
What's new in SQLAlchemy 0.5?
=============================
.. admonition:: About this Document
This document describes changes between SQLAlchemy version 0.4,
last released October 12, 2008, and SQLAlchemy version 0.5,
last released January 16, 2010.
Document date: August 4, 2009
This guide documents API changes which affect users
migrating their applications from the 0.4 series of
SQLAlchemy to 0.5. It's also recommended for those working
from `Essential SQLAlchemy
`_, which only
covers 0.4 and seems to even have some old 0.3isms in it.
Note that SQLAlchemy 0.5 removes many behaviors which were
deprecated throughout the span of the 0.4 series, and also
deprecates more behaviors specific to 0.4.
Major Documentation Changes
===========================
Some sections of the documentation have been completely
rewritten and can serve as an introduction to new ORM
features. The ``Query`` and ``Session`` objects in
particular have some distinct differences in API and
behavior which fundamentally change many of the basic ways
things are done, particularly with regards to constructing
highly customized ORM queries and dealing with stale session
state, commits and rollbacks.
* `ORM Tutorial
`_
* `Session Documentation
`_
Deprecations Source
===================
Another source of information is documented within a series
of unit tests illustrating up to date usages of some common
``Query`` patterns; this file can be viewed at
[source:sqlalchemy/trunk/test/orm/test_deprecations.py].
Requirements Changes
====================
* Python 2.4 or higher is required. The SQLAlchemy 0.4 line
is the last version with Python 2.3 support.
Object Relational Mapping
=========================
* **Column level expressions within Query.** - as detailed
in the `tutorial
`_,
``Query`` has the capability to create specific SELECT
statements, not just those against full rows:
::
session.query(User.name, func.count(Address.id).label("numaddresses")).join(Address).group_by(User.name)
The tuples returned by any multi-column/entity query are
*named*' tuples:
::
for row in session.query(User.name, func.count(Address.id).label('numaddresses')).join(Address).group_by(User.name):
print("name", row.name, "number", row.numaddresses)
``Query`` has a ``statement`` accessor, as well as a
``subquery()`` method which allow ``Query`` to be used to
create more complex combinations:
::
subq = session.query(Keyword.id.label('keyword_id')).filter(Keyword.name.in_(['beans', 'carrots'])).subquery()
recipes = session.query(Recipe).filter(exists().
where(Recipe.id==recipe_keywords.c.recipe_id).
where(recipe_keywords.c.keyword_id==subq.c.keyword_id)
)
* **Explicit ORM aliases are recommended for aliased joins**
- The ``aliased()`` function produces an "alias" of a
class, which allows fine-grained control of aliases in
conjunction with ORM queries. While a table-level alias
(i.e. ``table.alias()``) is still usable, an ORM level
alias retains the semantics of the ORM mapped object which
is significant for inheritance mappings, options, and
other scenarios. E.g.:
::
Friend = aliased(Person)
session.query(Person, Friend).join((Friend, Person.friends)).all()
* **query.join() greatly enhanced.** - You can now specify
the target and ON clause for a join in multiple ways. A
target class alone can be provided where SQLA will attempt
to form a join to it via foreign key in the same way as
``table.join(someothertable)``. A target and an explicit
ON condition can be provided, where the ON condition can
be a ``relation()`` name, an actual class descriptor, or a
SQL expression. Or the old way of just a ``relation()``
name or class descriptor works too. See the ORM tutorial
which has several examples.
* **Declarative is recommended for applications which don't
require (and don't prefer) abstraction between tables and
mappers** - The [/docs/05/reference/ext/declarative.html
Declarative] module, which is used to combine the
expression of ``Table``, ``mapper()``, and user defined
class objects together, is highly recommended as it
simplifies application configuration, ensures the "one
mapper per class" pattern, and allows the full range of
configuration available to distinct ``mapper()`` calls.
Separate ``mapper()`` and ``Table`` usage is now referred
to as "classical SQLAlchemy usage" and of course is freely
mixable with declarative.
* **The .c. attribute has been removed** from classes (i.e.
``MyClass.c.somecolumn``). As is the case in 0.4, class-
level properties are usable as query elements, i.e.
``Class.c.propname`` is now superseded by
``Class.propname``, and the ``c`` attribute continues to
remain on ``Table`` objects where they indicate the
namespace of ``Column`` objects present on the table.
To get at the Table for a mapped class (if you didn't keep
it around already):
::
table = class_mapper(someclass).mapped_table
Iterate through columns:
::
for col in table.c:
print(col)
Work with a specific column:
::
table.c.somecolumn
The class-bound descriptors support the full set of Column
operators as well as the documented relation-oriented
operators like ``has()``, ``any()``, ``contains()``, etc.
The reason for the hard removal of ``.c.`` is that in 0.5,
class-bound descriptors carry potentially different
meaning, as well as information regarding class mappings,
versus plain ``Column`` objects - and there are use cases
where you'd specifically want to use one or the other.
Generally, using class-bound descriptors invokes a set of
mapping/polymorphic aware translations, and using table-
bound columns does not. In 0.4, these translations were
applied across the board to all expressions, but 0.5
differentiates completely between columns and mapped
descriptors, only applying translations to the latter. So
in many cases, particularly when dealing with joined table
inheritance configurations as well as when using
``query()``, ``Class.propname`` and
``table.c.colname`` are not interchangeable.
For example, ``session.query(users.c.id, users.c.name)``
is different versus ``session.query(User.id, User.name)``;
in the latter case, the ``Query`` is aware of the mapper
in use and further mapper-specific operations like
``query.join()``, ``query.with_parent()`` etc.
may be used, but in the former case cannot. Additionally,
in polymorphic inheritance scenarios, the class-bound
descriptors refer to the columns present in the
polymorphic selectable in use, not necessarily the table
column which directly corresponds to the descriptor. For
example, a set of classes related by joined-table
inheritance to the ``person`` table along the
``person_id`` column of each table will all have their
``Class.person_id`` attribute mapped to the ``person_id``
column in ``person``, and not their subclass table.
Version 0.4 would map this behavior onto table-bound
``Column`` objects automatically. In 0.5, this automatic
conversion has been removed, so that you in fact *can* use
table-bound columns as a means to override the
translations which occur with polymorphic querying; this
allows ``Query`` to be able to create optimized selects
among joined-table or concrete-table inheritance setups,
as well as portable subqueries, etc.
* **Session Now Synchronizes Automatically with
Transactions.** Session now synchronizes against the
transaction automatically by default, including autoflush
and autoexpire. A transaction is present at all times
unless disabled using the ``autocommit`` option. When all
three flags are set to their default, the Session recovers
gracefully after rollbacks and it's very difficult to get
stale data into the session. See the new Session
documentation for details.
* **Implicit Order By Is Removed**. This will impact ORM
users who rely upon SA's "implicit ordering" behavior,
which states that all Query objects which don't have an
``order_by()`` will ORDER BY the "id" or "oid" column of
the primary mapped table, and all lazy/eagerly loaded
collections apply a similar ordering. In 0.5, automatic
ordering must be explicitly configured on ``mapper()`` and
``relation()`` objects (if desired), or otherwise when
using ``Query``.
To convert an 0.4 mapping to 0.5, such that its ordering
behavior will be extremely similar to 0.4 or previous, use
the ``order_by`` setting on ``mapper()`` and
``relation()``:
::
mapper(User, users, properties={
'addresses':relation(Address, order_by=addresses.c.id)
}, order_by=users.c.id)
To set ordering on a backref, use the ``backref()``
function:
::
'keywords':relation(Keyword, secondary=item_keywords,
order_by=keywords.c.name, backref=backref('items', order_by=items.c.id))
Using declarative ? To help with the new ``order_by``
requirement, ``order_by`` and friends can now be set using
strings which are evaluated in Python later on (this works
**only** with declarative, not plain mappers):
::
class MyClass(MyDeclarativeBase):
...
'addresses':relation("Address", order_by="Address.id")
It's generally a good idea to set ``order_by`` on
``relation()s`` which load list-based collections of
items, since that ordering cannot otherwise be affected.
Other than that, the best practice is to use
``Query.order_by()`` to control ordering of the primary
entities being loaded.
* **Session is now
autoflush=True/autoexpire=True/autocommit=False.** - To
set it up, just call ``sessionmaker()`` with no arguments.
The name ``transactional=True`` is now
``autocommit=False``. Flushes occur upon each query
issued (disable with ``autoflush=False``), within each
``commit()`` (as always), and before each
``begin_nested()`` (so rolling back to the SAVEPOINT is
meaningful). All objects are expired after each
``commit()`` and after each ``rollback()``. After
rollback, pending objects are expunged, deleted objects
move back to persistent. These defaults work together
very nicely and there's really no more need for old
techniques like ``clear()`` (which is renamed to
``expunge_all()`` as well).
P.S.: sessions are now reusable after a ``rollback()``.
Scalar and collection attribute changes, adds and deletes
are all rolled back.
* **session.add() replaces session.save(), session.update(),
session.save_or_update().** - the
``session.add(someitem)`` and ``session.add_all([list of
items])`` methods replace ``save()``, ``update()``, and
``save_or_update()``. Those methods will remain
deprecated throughout 0.5.
* **backref configuration made less verbose.** - The
``backref()`` function now uses the ``primaryjoin`` and
``secondaryjoin`` arguments of the forwards-facing
``relation()`` when they are not explicitly stated. It's
no longer necessary to specify
``primaryjoin``/``secondaryjoin`` in both directions
separately.
* **Simplified polymorphic options.** - The ORM's
"polymorphic load" behavior has been simplified. In 0.4,
mapper() had an argument called ``polymorphic_fetch``
which could be configured as ``select`` or ``deferred``.
This option is removed; the mapper will now just defer any
columns which were not present in the SELECT statement.
The actual SELECT statement used is controlled by the
``with_polymorphic`` mapper argument (which is also in 0.4
and replaces ``select_table``), as well as the
``with_polymorphic()`` method on ``Query`` (also in 0.4).
An improvement to the deferred loading of inheriting
classes is that the mapper now produces the "optimized"
version of the SELECT statement in all cases; that is, if
class B inherits from A, and several attributes only
present on class B have been expired, the refresh
operation will only include B's table in the SELECT
statement and will not JOIN to A.
* The ``execute()`` method on ``Session`` converts plain
strings into ``text()`` constructs, so that bind
parameters may all be specified as ":bindname" without
needing to call ``text()`` explicitly. If "raw" SQL is
desired here, use ``session.connection().execute("raw
text")``.
* ``session.Query().iterate_instances()`` has been renamed
to just ``instances()``. The old ``instances()`` method
returning a list instead of an iterator no longer exists.
If you were relying on that behavior, you should use
``list(your_query.instances())``.
Extending the ORM
=================
In 0.5 we're moving forward with more ways to modify and
extend the ORM. Heres a summary:
* **MapperExtension.** - This is the classic extension
class, which remains. Methods which should rarely be
needed are ``create_instance()`` and
``populate_instance()``. To control the initialization of
an object when it's loaded from the database, use the
``reconstruct_instance()`` method, or more easily the
``@reconstructor`` decorator described in the
documentation.
* **SessionExtension.** - This is an easy to use extension
class for session events. In particular, it provides
``before_flush()``, ``after_flush()`` and
``after_flush_postexec()`` methods. This usage is
recommended over ``MapperExtension.before_XXX`` in many
cases since within ``before_flush()`` you can modify the
flush plan of the session freely, something which cannot
be done from within ``MapperExtension``.
* **AttributeExtension.** - This class is now part of the
public API, and allows the interception of userland events
on attributes, including attribute set and delete
operations, and collection appends and removes. It also
allows the value to be set or appended to be modified.
The ``@validates`` decorator, described in the
documentation, provides a quick way to mark any mapped
attributes as being "validated" by a particular class
method.
* **Attribute Instrumentation Customization.** - An API is
provided for ambitious efforts to entirely replace
SQLAlchemy's attribute instrumentation, or just to augment
it in some cases. This API was produced for the purposes
of the Trellis toolkit, but is available as a public API.
Some examples are provided in the distribution in the
``/examples/custom_attributes`` directory.
Schema/Types
============
* **String with no length no longer generates TEXT, it
generates VARCHAR** - The ``String`` type no longer
magically converts into a ``Text`` type when specified
with no length. This only has an effect when CREATE TABLE
is issued, as it will issue ``VARCHAR`` with no length
parameter, which is not valid on many (but not all)
databases. To create a TEXT (or CLOB, i.e. unbounded
string) column, use the ``Text`` type.
* **PickleType() with mutable=True requires an __eq__()
method** - The ``PickleType`` type needs to compare values
when mutable=True. The method of comparing
``pickle.dumps()`` is inefficient and unreliable. If an
incoming object does not implement ``__eq__()`` and is
also not ``None``, the ``dumps()`` comparison is used but
a warning is raised. For types which implement
``__eq__()`` which includes all dictionaries, lists, etc.,
comparison will use ``==`` and is now reliable by default.
* **convert_bind_param() and convert_result_value() methods
of TypeEngine/TypeDecorator are removed.** - The O'Reilly
book unfortunately documented these methods even though
they were deprecated post 0.3. For a user-defined type
which subclasses ``TypeEngine``, the ``bind_processor()``
and ``result_processor()`` methods should be used for
bind/result processing. Any user defined type, whether
extending ``TypeEngine`` or ``TypeDecorator``, which uses
the old 0.3 style can be easily adapted to the new style
using the following adapter:
::
class AdaptOldConvertMethods(object):
"""A mixin which adapts 0.3-style convert_bind_param and
convert_result_value methods
"""
def bind_processor(self, dialect):
def convert(value):
return self.convert_bind_param(value, dialect)
return convert
def result_processor(self, dialect):
def convert(value):
return self.convert_result_value(value, dialect)
return convert
def convert_result_value(self, value, dialect):
return value
def convert_bind_param(self, value, dialect):
return value
To use the above mixin:
::
class MyType(AdaptOldConvertMethods, TypeEngine):
# ...
* The ``quote`` flag on ``Column`` and ``Table`` as well as
the ``quote_schema`` flag on ``Table`` now control quoting
both positively and negatively. The default is ``None``,
meaning let regular quoting rules take effect. When
``True``, quoting is forced on. When ``False``, quoting
is forced off.
* Column ``DEFAULT`` value DDL can now be more conveniently
specified with ``Column(..., server_default='val')``,
deprecating ``Column(..., PassiveDefault('val'))``.
``default=`` is now exclusively for Python-initiated
default values, and can coexist with server_default. A
new ``server_default=FetchedValue()`` replaces the
``PassiveDefault('')`` idiom for marking columns as
subject to influence from external triggers and has no DDL
side effects.
* SQLite's ``DateTime``, ``Time`` and ``Date`` types now
**only accept datetime objects, not strings** as bind
parameter input. If you'd like to create your own
"hybrid" type which accepts strings and returns results as
date objects (from whatever format you'd like), create a
``TypeDecorator`` that builds on ``String``. If you only
want string-based dates, just use ``String``.
* Additionally, the ``DateTime`` and ``Time`` types, when
used with SQLite, now represent the "microseconds" field
of the Python ``datetime.datetime`` object in the same
manner as ``str(datetime)`` - as fractional seconds, not a
count of microseconds. That is:
::
dt = datetime.datetime(2008, 6, 27, 12, 0, 0, 125) # 125 usec
# old way
'2008-06-27 12:00:00.125'
# new way
'2008-06-27 12:00:00.000125'
So if an existing SQLite file-based database intends to be
used across 0.4 and 0.5, you either have to upgrade the
datetime columns to store the new format (NOTE: please
test this, I'm pretty sure its correct):
::
UPDATE mytable SET somedatecol =
substr(somedatecol, 0, 19) || '.' || substr((substr(somedatecol, 21, -1) / 1000000), 3, -1);
or, enable "legacy" mode as follows:
::
from sqlalchemy.databases.sqlite import DateTimeMixin
DateTimeMixin.__legacy_microseconds__ = True
Connection Pool no longer threadlocal by default
================================================
0.4 has an unfortunate default setting of
"pool_threadlocal=True", leading to surprise behavior when,
for example, using multiple Sessions within a single thread.
This flag is now off in 0.5. To re-enable 0.4's behavior,
specify ``pool_threadlocal=True`` to ``create_engine()``, or
alternatively use the "threadlocal" strategy via
``strategy="threadlocal"``.
\*args Accepted, \*args No Longer Accepted
==========================================
The policy with ``method(\*args)`` vs. ``method([args])``
is, if the method accepts a variable-length set of items
which represent a fixed structure, it takes ``\*args``. If
the method accepts a variable-length set of items that are
data-driven, it takes ``[args]``.
* The various Query.options() functions ``eagerload()``,
``eagerload_all()``, ``lazyload()``, ``contains_eager()``,
``defer()``, ``undefer()`` all accept variable-length
``\*keys`` as their argument now, which allows a path to
be formulated using descriptors, ie.:
::
query.options(eagerload_all(User.orders, Order.items, Item.keywords))
A single array argument is still accepted for backwards
compatibility.
* Similarly, the ``Query.join()`` and ``Query.outerjoin()``
methods accept a variable length \*args, with a single
array accepted for backwards compatibility:
::
query.join('orders', 'items')
query.join(User.orders, Order.items)
* the ``in_()`` method on columns and similar only accepts a
list argument now. It no longer accepts ``\*args``.
Removed
=======
* **entity_name** - This feature was always problematic and
rarely used. 0.5's more deeply fleshed out use cases
revealed further issues with ``entity_name`` which led to
its removal. If different mappings are required for a
single class, break the class into separate subclasses and
map them separately. An example of this is at
[wiki:UsageRecipes/EntityName]. More information
regarding rationale is described at http://groups.google.c
om/group/sqlalchemy/browse_thread/thread/9e23a0641a88b96d?
hl=en .
* **get()/load() cleanup**
The ``load()`` method has been removed. Its
functionality was kind of arbitrary and basically copied
from Hibernate, where it's also not a particularly
meaningful method.
To get equivalent functionality:
::
x = session.query(SomeClass).populate_existing().get(7)
``Session.get(cls, id)`` and ``Session.load(cls, id)``
have been removed. ``Session.get()`` is redundant vs.
``session.query(cls).get(id)``.
``MapperExtension.get()`` is also removed (as is
``MapperExtension.load()``). To override the
functionality of ``Query.get()``, use a subclass:
::
class MyQuery(Query):
def get(self, ident):
# ...
session = sessionmaker(query_cls=MyQuery)()
ad1 = session.query(Address).get(1)
* ``sqlalchemy.orm.relation()``
The following deprecated keyword arguments have been
removed:
foreignkey, association, private, attributeext, is_backref
In particular, ``attributeext`` is replaced with
``extension`` - the ``AttributeExtension`` class is now in
the public API.
* ``session.Query()``
The following deprecated functions have been removed:
list, scalar, count_by, select_whereclause, get_by,
select_by, join_by, selectfirst, selectone, select,
execute, select_statement, select_text, join_to, join_via,
selectfirst_by, selectone_by, apply_max, apply_min,
apply_avg, apply_sum
Additionally, the ``id`` keyword argument to ``join()``,
``outerjoin()``, ``add_entity()`` and ``add_column()`` has
been removed. To target table aliases in ``Query`` to
result columns, use the ``aliased`` construct:
::
from sqlalchemy.orm import aliased
address_alias = aliased(Address)
print(session.query(User, address_alias).join((address_alias, User.addresses)).all())
* ``sqlalchemy.orm.Mapper``
* instances()
* get_session() - this method was not very noticeable, but
had the effect of associating lazy loads with a
particular session even if the parent object was
entirely detached, when an extension such as
``scoped_session()`` or the old ``SessionContextExt``
was used. It's possible that some applications which
relied upon this behavior will no longer work as
expected; but the better programming practice here is
to always ensure objects are present within sessions if
database access from their attributes are required.
* ``mapper(MyClass, mytable)``
Mapped classes no are longer instrumented with a "c" class
attribute; e.g. ``MyClass.c``
* ``sqlalchemy.orm.collections``
The _prepare_instrumentation alias for
prepare_instrumentation has been removed.
* ``sqlalchemy.orm``
Removed the ``EXT_PASS`` alias of ``EXT_CONTINUE``.
* ``sqlalchemy.engine``
The alias from ``DefaultDialect.preexecute_sequences`` to
``.preexecute_pk_sequences`` has been removed.
The deprecated engine_descriptors() function has been
removed.
* ``sqlalchemy.ext.activemapper``
Module removed.
* ``sqlalchemy.ext.assignmapper``
Module removed.
* ``sqlalchemy.ext.associationproxy``
Pass-through of keyword args on the proxy's
``.append(item, \**kw)`` has been removed and is now
simply ``.append(item)``
* ``sqlalchemy.ext.selectresults``,
``sqlalchemy.mods.selectresults``
Modules removed.
* ``sqlalchemy.ext.declarative``
``declared_synonym()`` removed.
* ``sqlalchemy.ext.sessioncontext``
Module removed.
* ``sqlalchemy.log``
The ``SADeprecationWarning`` alias to
``sqlalchemy.exc.SADeprecationWarning`` has been removed.
* ``sqlalchemy.exc``
``exc.AssertionError`` has been removed and usage replaced
by the Python built-in of the same name.
* ``sqlalchemy.databases.mysql``
The deprecated ``get_version_info`` dialect method has
been removed.
Renamed or Moved
================
* ``sqlalchemy.exceptions`` is now ``sqlalchemy.exc``
The module may still be imported under the old name until
0.6.
* ``FlushError``, ``ConcurrentModificationError``,
``UnmappedColumnError`` -> sqlalchemy.orm.exc
These exceptions moved to the orm package. Importing
'sqlalchemy.orm' will install aliases in sqlalchemy.exc
for compatibility until 0.6.
* ``sqlalchemy.logging`` -> ``sqlalchemy.log``
This internal module was renamed. No longer needs to be
special cased when packaging SA with py2app and similar
tools that scan imports.
* ``session.Query().iterate_instances()`` ->
``session.Query().instances()``.
Deprecated
==========
* ``Session.save()``, ``Session.update()``,
``Session.save_or_update()``
All three replaced by ``Session.add()``
* ``sqlalchemy.PassiveDefault``
Use ``Column(server_default=...)`` Translates to
sqlalchemy.DefaultClause() under the hood.
* ``session.Query().iterate_instances()``. It has been
renamed to ``instances()``.