Release: 2.0.38 | Release Date: February 6, 2025

# SQLAlchemy 2.0 Documentation

# What’s new in SQLAlchemy 0.5?

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(<columns>)`, `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(<propname>)`, `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 https://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()`.