# SQLAlchemy 0.9 Documentation

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Release: 0.9.3 | Release Date: February 19, 2014

# What’s New in SQLAlchemy 0.9?

About this Document

This document describes changes between SQLAlchemy version 0.8, undergoing maintenance releases as of May, 2013, and SQLAlchemy version 0.9, which is expected for release in late 2013.

Document last updated: January 8, 2014

## Introduction

This guide introduces what’s new in SQLAlchemy version 0.9, and also documents changes which affect users migrating their applications from the 0.8 series of SQLAlchemy to 0.9.

Please carefully review Behavioral Changes - ORM and Behavioral Changes - Core for potentially backwards-incompatible changes.

## Platform Support

### Targeting Python 2.6 and Up Now, Python 3 without 2to3

The first achievement of the 0.9 release is to remove the dependency on the 2to3 tool for Python 3 compatibility. To make this more straightforward, the lowest Python release targeted now is 2.6, which features a wide degree of cross-compatibility with Python 3. All SQLAlchemy modules and unit tests are now interpreted equally well with any Python interpreter from 2.6 forward, including the 3.1 and 3.2 interpreters.

#2671

### C Extensions Supported on Python 3

The C extensions have been ported to support Python 3 and now build in both Python 2 and Python 3 environments.

#2161

## Behavioral Changes - ORM

### Composite attributes are now returned as their object form when queried on a per-attribute basis

Using a `Query` in conjunction with a composite attribute now returns the object type maintained by that composite, rather than being broken out into individual columns. Using the mapping setup at Composite Column Types:

    >>> session.query(Vertex.start, Vertex.end).\
    ...     filter(Vertex.start == Point(3, 4)).all()
    [(Point(x=3, y=4), Point(x=5, y=6))]

This change is backwards-incompatible with code that expects the indivdual attribute to be expanded into individual columns. To get that behavior, use the `.clauses` accessor:

    >>> session.query(Vertex.start.clauses, Vertex.end.clauses).\
    ...     filter(Vertex.start == Point(3, 4)).all()
    [(3, 4, 5, 6)]

See also

Column Bundles for ORM queries

#2824

### `Query.select_from()` no longer applies the clause to corresponding entities

The `Query.select_from()` method has been popularized in recent versions as a means of controlling the first thing that a `Query` object “selects from”, typically for the purposes of controlling how a JOIN will render.

Consider the following example against the usual `User` mapping:

    select_stmt = select([User]).where(User.id == 7).alias()

    q = session.query(User).\
               join(select_stmt, User.id == select_stmt.c.id).\
               filter(User.name == 'ed')

The above statement predictably renders SQL like the following:

    SELECT "user".id AS user_id, "user".name AS user_name
    FROM "user" JOIN (SELECT "user".id AS id, "user".name AS name
    FROM "user"
    WHERE "user".id = :id_1) AS anon_1 ON "user".id = anon_1.id
    WHERE "user".name = :name_1

If we wanted to reverse the order of the left and right elements of the JOIN, the documentation would lead us to believe we could use `Query.select_from()` to do so:

    q = session.query(User).\
            select_from(select_stmt).\
            join(User, User.id == select_stmt.c.id).\
            filter(User.name == 'ed')

However, in version 0.8 and earlier, the above use of `Query.select_from()` would apply the `select_stmt` to replace the `User` entity, as it selects from the `user` table which is compatible with `User`:

    -- SQLAlchemy 0.8 and earlier...
    SELECT anon_1.id AS anon_1_id, anon_1.name AS anon_1_name
    FROM (SELECT "user".id AS id, "user".name AS name
    FROM "user"
    WHERE "user".id = :id_1) AS anon_1 JOIN "user" ON anon_1.id = anon_1.id
    WHERE anon_1.name = :name_1

The above statement is a mess, the ON clause refers `anon_1.id = anon_1.id`, our WHERE clause has been replaced with `anon_1` as well.

This behavior is quite intentional, but has a different use case from that which has become popular for `Query.select_from()`. The above behavior is now available by a new method known as `Query.select_entity_from()`. This is a lesser used behavior that in modern SQLAlchemy is roughly equivalent to selecting from a customized `aliased()` construct:

    select_stmt = select([User]).where(User.id == 7)
    user_from_stmt = aliased(User, select_stmt.alias())

    q = session.query(user_from_stmt).filter(user_from_stmt.name == 'ed')

So with SQLAlchemy 0.9, our query that selects from `select_stmt` produces the SQL we expect:

    -- SQLAlchemy 0.9
    SELECT "user".id AS user_id, "user".name AS user_name
    FROM (SELECT "user".id AS id, "user".name AS name
    FROM "user"
    WHERE "user".id = :id_1) AS anon_1 JOIN "user" ON "user".id = id
    WHERE "user".name = :name_1

The `Query.select_entity_from()` method will be available in SQLAlchemy 0.8.2, so applications which rely on the old behavior can transition to this method first, ensure all tests continue to function, then upgrade to 0.9 without issue.

#2736

### `viewonly=True` on `relationship()` prevents history from taking effect

The `viewonly` flag on `relationship()` is applied to prevent changes to the target attribute from having any effect within the flush process. This is achieved by eliminating the attribute from being considered during the flush. However, up until now, changes to the attribute would still register the parent object as “dirty” and trigger a potential flush. The change is that the `viewonly` flag now prevents history from being set for the target attribute as well. Attribute events like backrefs and user-defined events still continue to function normally.

The change is illustrated as follows:

    from sqlalchemy import Column, Integer, ForeignKey, create_engine
    from sqlalchemy.orm import backref, relationship, Session
    from sqlalchemy.ext.declarative import declarative_base
    from sqlalchemy import inspect

    Base = declarative_base()

    class A(Base):
        __tablename__ = 'a'
        id = Column(Integer, primary_key=True)

    class B(Base):
        __tablename__ = 'b'

        id = Column(Integer, primary_key=True)
        a_id = Column(Integer, ForeignKey('a.id'))
        a = relationship("A", backref=backref("bs", viewonly=True))

    e = create_engine("sqlite://")
    Base.metadata.create_all(e)

    a = A()
    b = B()

    sess = Session(e)
    sess.add_all([a, b])
    sess.commit()

    b.a = a

    assert b in sess.dirty

    # before 0.9.0
    # assert a in sess.dirty
    # assert inspect(a).attrs.bs.history.has_changes()

    # after 0.9.0
    assert a not in sess.dirty
    assert not inspect(a).attrs.bs.history.has_changes()

#2833

### Association Proxy SQL Expression Improvements and Fixes

The `==` and `!=` operators as implemented by an association proxy that refers to a scalar value on a scalar relationship now produces a more complete SQL expression, intended to take into account the “association” row being present or not when the comparison is against `None`.

Consider this mapping:

    class A(Base):
        __tablename__ = 'a'

        id = Column(Integer, primary_key=True)

        b_id = Column(Integer, ForeignKey('b.id'), primary_key=True)
        b = relationship("B")
        b_value = association_proxy("b", "value")

    class B(Base):
        __tablename__ = 'b'
        id = Column(Integer, primary_key=True)
        value = Column(String)

Up through 0.8, a query like the following:

    s.query(A).filter(A.b_value == None).all()

would produce:

    SELECT a.id AS a_id, a.b_id AS a_b_id
    FROM a
    WHERE EXISTS (SELECT 1
    FROM b
    WHERE b.id = a.b_id AND b.value IS NULL)

In 0.9, it now produces:

    SELECT a.id AS a_id, a.b_id AS a_b_id
    FROM a
    WHERE (EXISTS (SELECT 1
    FROM b
    WHERE b.id = a.b_id AND b.value IS NULL)) OR a.b_id IS NULL

The difference being, it not only checks `b.value`, it also checks if `a` refers to no `b` row at all. This will return different results versus prior versions, for a system that uses this type of comparison where some parent rows have no association row.

More critically, a correct expression is emitted for `A.b_value != None`. In 0.8, this would return `True` for `A` rows that had no `b`:

    SELECT a.id AS a_id, a.b_id AS a_b_id
    FROM a
    WHERE NOT (EXISTS (SELECT 1
    FROM b
    WHERE b.id = a.b_id AND b.value IS NULL))

Now in 0.9, the check has been reworked so that it ensures the A.b_id row is present, in addition to `B.value` being non-NULL:

    SELECT a.id AS a_id, a.b_id AS a_b_id
    FROM a
    WHERE EXISTS (SELECT 1
    FROM b
    WHERE b.id = a.b_id AND b.value IS NOT NULL)

In addition, the `has()` operator is enhanced such that you can call it against a scalar column value with no criterion only, and it will produce criteria that checks for the association row being present or not:

    s.query(A).filter(A.b_value.has()).all()

output:

    SELECT a.id AS a_id, a.b_id AS a_b_id
    FROM a
    WHERE EXISTS (SELECT 1
    FROM b
    WHERE b.id = a.b_id)

This is equivalent to `A.b.has()`, but allows one to query against `b_value` directly.

#2751

### Association Proxy Missing Scalar returns None

An association proxy from a scalar attribute to a scalar will now return `None` if the proxied object isn’t present. This is consistent with the fact that missing many-to-ones return None in SQLAlchemy, so should the proxied value. E.g.:

    from sqlalchemy import *
    from sqlalchemy.orm import *
    from sqlalchemy.ext.declarative import declarative_base
    from sqlalchemy.ext.associationproxy import association_proxy

    Base = declarative_base()

    class A(Base):
        __tablename__ = 'a'

        id = Column(Integer, primary_key=True)
        b = relationship("B", uselist=False)

        bname = association_proxy("b", "name")

    class B(Base):
        __tablename__ = 'b'

        id = Column(Integer, primary_key=True)
        a_id = Column(Integer, ForeignKey('a.id'))
        name = Column(String)

    a1 = A()

    # this is how m2o's always have worked
    assert a1.b is None

    # but prior to 0.9, this would raise AttributeError,
    # now returns None just like the proxied value.
    assert a1.bname is None

#2810

### attributes.get_history() will query from the DB by default if value not present

A bugfix regarding `attributes.get_history()` allows a column-based attribute to query out to the database for an unloaded value, assuming the `passive` flag is left at its default of `PASSIVE_OFF`. Previously, this flag would not be honored. Additionally, a new method `AttributeState.load_history()` is added to complement the `AttributeState.history` attribute, which will emit loader callables for an unloaded attribute.

This is a small change demonstrated as follows:

    from sqlalchemy import Column, Integer, String, create_engine, inspect
    from sqlalchemy.orm import Session, attributes
    from sqlalchemy.ext.declarative import declarative_base

    Base = declarative_base()

    class A(Base):
        __tablename__ = 'a'
        id = Column(Integer, primary_key=True)
        data = Column(String)

    e = create_engine("sqlite://", echo=True)
    Base.metadata.create_all(e)

    sess = Session(e)

    a1 = A(data='a1')
    sess.add(a1)
    sess.commit()  # a1 is now expired

    # history doesn't emit loader callables
    assert inspect(a1).attrs.data.history == (None, None, None)

    # in 0.8, this would fail to load the unloaded state.
    assert attributes.get_history(a1, 'data') == ((), ['a1',], ())

    # load_history() is now equiavlent to get_history() with
    # passive=PASSIVE_OFF ^ INIT_OK
    assert inspect(a1).attrs.data.load_history() == ((), ['a1',], ())

#2787

## Behavioral Changes - Core

### `None` can no longer be used as a “partial AND” constructor

`None` can no longer be used as the “backstop” to form an AND condition piecemeal. This pattern was not a documented pattern even though some SQLAlchemy internals made use of it:

    condition = None

    for cond in conditions:
        condition = condition & cond

    if condition is not None:
        stmt = stmt.where(condition)

The above sequence, when `conditions` is non-empty, will on 0.9 produce `SELECT .. WHERE <condition> AND NULL`. The `None` is no longer implicitly ignored, and is instead consistent with when `None` is interpreted in other contexts besides that of a conjunction.

The correct code for both 0.8 and 0.9 should read:

    from sqlalchemy.sql import and_

    if conditions:
        stmt = stmt.where(and_(*conditions))

Another variant that works on all backends on 0.9, but on 0.8 only works on backends that support boolean constants:

    from sqlalchemy.sql import true

    condition = true()

    for cond in conditions:
        condition = cond & condition

    stmt = stmt.where(condition)

On 0.8, this will produce a SELECT statement that always has `AND true` in the WHERE clause, which is not accepted by backends that don’t support boolean constants (MySQL, MSSQL). On 0.9, the `true` constant will be dropped within an `and_()` conjunction.

See also

Improved rendering of Boolean constants, NULL constants, conjunctions

### The “password” portion of a `create_engine()` no longer considers the `+` sign as an encoded space

For whatever reason, the Python function `unquote_plus()` was applied to the “password” field of a URL, which is an incorrect application of the encoding rules described in RFC 1738 in that it escaped spaces as plus signs. The stringiciation of a URL now only encodes ”:”, “@”, or “/” and nothing else, and is now applied to both the `username` and `password` fields (previously it only applied to the password). On parsing, encoded characters are converted, but plus signs and spaces are passed through as is:

    # password: "pass word + other:words"
    dbtype://user:pass word + other%3Awords@host/dbname

    # password: "apples/oranges"
    dbtype://username:apples%2Foranges@hostspec/database

    # password: "apples@oranges@@"
    dbtype://username:apples%40oranges%40%40@hostspec/database

    # password: '', username is "username@"
    dbtype://username%40:@hostspec/database

#2873

### The precedence rules for COLLATE have been changed

Previously, an expression like the following:

    print (column('x') == 'somevalue').collate("en_EN")

would produce an expression like this:

    -- 0.8 behavior
    (x = :x_1) COLLATE en_EN

The above is misunderstood by MSSQL and is generally not the syntax suggested for any database. The expression will now produce the syntax illustrated by that of most database documentation:

    -- 0.9 behavior
    x = :x_1 COLLATE en_EN

The potentially backwards incompatible change arises if the `collate()` operator is being applied to the right-hand column, as follows:

    print column('x') == literal('somevalue').collate("en_EN")

In 0.8, this produces:

    x = :param_1 COLLATE en_EN

However in 0.9, will now produce the more accurate, but probably not what you want, form of:

    x = (:param_1 COLLATE en_EN)

The `ColumnOperators.collate()` operator now works more appropriately within an `ORDER BY` expression as well, as a specific precedence has been given to the `ASC` and `DESC` operators which will again ensure no parentheses are generated:

    >>> # 0.8
    >>> print column('x').collate('en_EN').desc()
    (x COLLATE en_EN) DESC

    >>> # 0.9
    >>> print column('x').collate('en_EN').desc()
    x COLLATE en_EN DESC

#2879

### Postgresql CREATE TYPE <x> AS ENUM now applies quoting to values

The `postgresql.ENUM` type will now apply escaping to single quote signs within the enumerated values:

    >>> from sqlalchemy.dialects import postgresql
    >>> type = postgresql.ENUM('one', 'two', "three's", name="myenum")
    >>> from sqlalchemy.dialects.postgresql import base
    >>> print base.CreateEnumType(type).compile(dialect=postgresql.dialect())
    CREATE TYPE myenum AS ENUM ('one','two','three''s')

Existing workarounds which already escape single quote signs will need to be modified, else they will now double-escape.

#2878

## New Features

### Event Removal API

Events established using `event.listen()` or `event.listens_for()` can now be removed using the new `event.remove()` function. The `target`, `identifier` and `fn` arguments sent to `event.remove()` need to match exactly those which were sent for listening, and the event will be removed from all locations in which it had been established:

    @event.listens_for(MyClass, "before_insert", propagate=True)
    def my_before_insert(mapper, connection, target):
        """listen for before_insert"""
        # ...

    event.remove(MyClass, "before_insert", my_before_insert)

In the example above, the `propagate=True` flag is set. This means `my_before_insert()` is established as a listener for `MyClass` as well as all subclasses of `MyClass`. The system tracks everywhere that the `my_before_insert()` listener function had been placed as a result of this call and removes it as a result of calling `event.remove()`.

The removal system uses a registry to associate arguments passed to `event.listen()` with collections of event listeners, which are in many cases wrapped versions of the original user-supplied function. This registry makes heavy use of weak references in order to allow all the contained contents, such as listener targets, to be garbage collected when they go out of scope.

#2268

### New Query Options API; `load_only()` option

The system of loader options such as `orm.joinedload()`, `orm.subqueryload()`, `orm.lazyload()`, `orm.defer()`, etc. all build upon a new system known as `Load`. `Load` provides a “method chained” (a.k.a. generative) approach to loader options, so that instead of joining together long paths using dots or multiple attribute names, an explicit loader style is given for each path.

While the new way is slightly more verbose, it is simpler to understand in that there is no ambiguity in what options are being applied to which paths; it simplifies the method signatures of the options and provides greater flexibility particularly for column-based options. The old systems are to remain functional indefinitely as well and all styles can be mixed.

Old Way

To set a certain style of loading along every link in a multi-element path, the `_all()` option has to be used:

    query(User).options(joinedload_all("orders.items.keywords"))

New Way

Loader options are now chainable, so the same `joinedload(x)` method is applied equally to each link, without the need to keep straight between `joinedload()` and `joinedload_all()`:

    query(User).options(joinedload("orders").joinedload("items").joinedload("keywords"))

Old Way

Setting an option on path that is based on a subclass requires that all links in the path be spelled out as class bound attributes, since the `PropComparator.of_type()` method needs to be called:

    session.query(Company).\
        options(
            subqueryload_all(
                Company.employees.of_type(Engineer),
                Engineer.machines
            )
        )

New Way

Only those elements in the path that actually need `PropComparator.of_type()` need to be set as a class-bound attribute, string-based names can be resumed afterwards:

    session.query(Company).\
        options(
            subqueryload(Company.employees.of_type(Engineer)).
            subqueryload("machines")
            )
        )

Old Way

Setting the loader option on the last link in a long path uses a syntax that looks a lot like it should be setting the option for all links in the path, causing confusion:

    query(User).options(subqueryload("orders.items.keywords"))

New Way

A path can now be spelled out using `defaultload()` for entries in the path where the existing loader style should be unchanged. More verbose but the intent is clearer:

    query(User).options(defaultload("orders").defaultload("items").subqueryload("keywords"))

The dotted style can still be taken advantage of, particularly in the case of skipping over several path elements:

    query(User).options(defaultload("orders.items").subqueryload("keywords"))

Old Way

The `defer()` option on a path needed to be spelled out with the full path for each column:

    query(User).options(defer("orders.description"), defer("orders.isopen"))

New Way

A single `Load` object that arrives at the target path can have `Load.defer()` called upon it repeatedly:

    query(User).options(defaultload("orders").defer("description").defer("isopen"))

#### The Load Class

The `Load` class can be used directly to provide a “bound” target, especially when multiple parent entities are present:

    from sqlalchemy.orm import Load

    query(User, Address).options(Load(Address).joinedload("entries"))

#### Load Only

A new option `load_only()` achieves a “defer everything but” style of load, loading only the given columns and deferring the rest:

    from sqlalchemy.orm import load_only

    query(User).options(load_only("name", "fullname"))

    # specify explicit parent entity
    query(User, Address).options(Load(User).load_only("name", "fullname"))

    # specify path
    query(User).options(joinedload(User.addresses).load_only("email_address"))

#### Class-specific Wildcards

Using `Load`, a wildcard may be used to set the loading for all relationships (or perhaps columns) on a given entity, without affecting any others:

    # lazyload all User relationships
    query(User).options(Load(User).lazyload("*"))

    # undefer all User columns
    query(User).options(Load(User).undefer("*"))

    # lazyload all Address relationships
    query(User).options(defaultload(User.addresses).lazyload("*"))

    # undefer all Address columns
    query(User).options(defaultload(User.addresses).undefer("*"))

#1418

### New `text()` Capabilities

The `text()` construct gains new methods:

- `TextClause.bindparams()` allows bound parameter types and values to be set flexibly:

      # setup values
      stmt = text("SELECT id, name FROM user "
            "WHERE name=:name AND timestamp=:timestamp").\
            bindparams(name="ed", timestamp=datetime(2012, 11, 10, 15, 12, 35))

      # setup types and/or values
      stmt = text("SELECT id, name FROM user "
            "WHERE name=:name AND timestamp=:timestamp").\
            bindparams(
                bindparam("name", value="ed"),
                bindparam("timestamp", type_=DateTime()
            ).bindparam(timestamp=datetime(2012, 11, 10, 15, 12, 35))

- `TextClause.columns()` supersedes the `typemap` option of `text()`, returning a new construct `TextAsFrom`:

      # turn a text() into an alias(), with a .c. collection:
      stmt = text("SELECT id, name FROM user").columns(id=Integer, name=String)
      stmt = stmt.alias()

      stmt = select([addresses]).select_from(
                    addresses.join(stmt), addresses.c.user_id == stmt.c.id)

      # or into a cte():
      stmt = text("SELECT id, name FROM user").columns(id=Integer, name=String)
      stmt = stmt.cte("x")

      stmt = select([addresses]).select_from(
                    addresses.join(stmt), addresses.c.user_id == stmt.c.id)

#2877

### INSERT from SELECT

After literally years of pointless procrastination this relatively minor syntactical feature has been added, and is also backported to 0.8.3, so technically isn’t “new” in 0.9. A `select()` construct or other compatible construct can be passed to the new method `Insert.from_select()` where it will be used to render an `INSERT .. SELECT` construct:

    >>> from sqlalchemy.sql import table, column
    >>> t1 = table('t1', column('a'), column('b'))
    >>> t2 = table('t2', column('x'), column('y'))
    >>> print(t1.insert().from_select(['a', 'b'], t2.select().where(t2.c.y == 5)))
    INSERT INTO t1 (a, b) SELECT t2.x, t2.y
    FROM t2
    WHERE t2.y = :y_1

The construct is smart enough to also accommodate ORM objects such as classes and `Query` objects:

    s = Session()
    q = s.query(User.id, User.name).filter_by(name='ed')
    ins = insert(Address).from_select((Address.id, Address.email_address), q)

rendering:

    INSERT INTO addresses (id, email_address)
    SELECT users.id AS users_id, users.name AS users_name
    FROM users WHERE users.name = :name_1

#722

### New FOR UPDATE support on `select()`, `Query()`

An attempt is made to simplify the specification of the `FOR UPDATE` clause on `SELECT` statements made within Core and ORM, and support is added for the `FOR UPDATE OF` SQL supported by Postgresql and Oracle.

Using the core `GenerativeSelect.with_for_update()`, options like `FOR SHARE` and `NOWAIT` can be specified individually, rather than linking to arbitrary string codes:

    stmt = select([table]).with_for_update(read=True, nowait=True, of=table)

On Posgtresql the above statement might render like:

    SELECT table.a, table.b FROM table FOR SHARE OF table NOWAIT

The `Query` object gains a similar method `Query.with_for_update()` which behaves in the same way. This method supersedes the existing `Query.with_lockmode()` method, which translated `FOR UPDATE` clauses using a different system. At the moment, the “lockmode” string argument is still accepted by the `Session.refresh()` method.

### Floating Point String-Conversion Precision Configurable for Native Floating Point Types

The conversion which SQLAlchemy does whenever a DBAPI returns a Python floating point type which is to be converted into a Python `Decimal()` necessarily involves an intermediary step which converts the floating point value to a string. The scale used for this string conversion was previously hardcoded to 10, and is now configurable. The setting is available on both the `Numeric` as well as the `Float` type, as well as all SQL- and dialect-specific descendant types, using the parameter `decimal_return_scale`. If the type supports a `.scale` parameter, as is the case with `Numeric` and some float types such as `mysql.DOUBLE`, the value of `.scale` is used as the default for `.decimal_return_scale` if it is not otherwise specified. If both `.scale` and `.decimal_return_scale` are absent, then the default of 10 takes place. E.g.:

    from sqlalchemy.dialects.mysql import DOUBLE
    import decimal

    data = Table('data', metadata,
        Column('double_value',
                    mysql.DOUBLE(decimal_return_scale=12, asdecimal=True))
    )

    conn.execute(
        data.insert(),
        double_value=45.768392065789,
    )
    result = conn.scalar(select([data.c.double_value]))

    # previously, this would typically be Decimal("45.7683920658"),
    # e.g. trimmed to 10 decimal places

    # now we get 12, as requested, as MySQL can support this
    # much precision for DOUBLE
    assert result == decimal.Decimal("45.768392065789")

#2867

### Column Bundles for ORM queries

The `Bundle` allows for querying of sets of columns, which are then grouped into one name under the tuple returned by the query. The initial purposes of `Bundle` are 1. to allow “composite” ORM columns to be returned as a single value in a column-based result set, rather than expanding them out into individual columns and 2. to allow the creation of custom result-set constructs within the ORM, using ad-hoc columns and return types, without involving the more heavyweight mechanics of mapped classes.

See also

Composite attributes are now returned as their object form when queried on a per-attribute basis

Column Bundles

#2824

### Server Side Version Counting

The versioning feature of the ORM (now also documented at Configuring a Version Counter) can now make use of server-side version counting schemes, such as those produced by triggers or database system columns, as well as conditional programmatic schemes outside of the version_id_counter function itself. By providing the value `False` to the `version_id_generator` parameter, the ORM will use the already-set version identifier, or alternatively fetch the version identifier from each row at the same time the INSERT or UPDATE is emitted. When using a server-generated version identifier, it is strongly recommended that this feature be used only on a backend with strong RETURNING support (Postgresql, SQL Server; Oracle also supports RETURNING but the cx_oracle driver has only limited support), else the additional SELECT statements will add significant performance overhead. The example provided at Server Side Version Counters illustrates the usage of the Postgresql `xmin` system column in order to integrate it with the ORM’s versioning feature.

See also

Server Side Version Counters

#2793

### `include_backrefs=False` option for `@validates`

The `validates()` function now accepts an option `include_backrefs=True`, which will bypass firing the validator for the case where the event initiated from a backref:

    from sqlalchemy import Column, Integer, ForeignKey
    from sqlalchemy.orm import relationship, validates
    from sqlalchemy.ext.declarative import declarative_base

    Base = declarative_base()

    class A(Base):
        __tablename__ = 'a'

        id = Column(Integer, primary_key=True)
        bs = relationship("B", backref="a")

        @validates("bs")
        def validate_bs(self, key, item):
            print("A.bs validator")
            return item

    class B(Base):
        __tablename__ = 'b'

        id = Column(Integer, primary_key=True)
        a_id = Column(Integer, ForeignKey('a.id'))

        @validates("a", include_backrefs=False)
        def validate_a(self, key, item):
            print("B.a validator")
            return item

    a1 = A()
    a1.bs.append(B())  # prints only "A.bs validator"

#1535

### Postgresql JSON Type

The Postgresql dialect now features a `postgresql.JSON` type to complement the `postgresql.HSTORE` type.

See also

`postgresql.JSON`

#2581

### Automap Extension

A new extension is added in 0.9.1 known as `sqlalchemy.ext.automap`. This is an experimental extension which expands upon the functionality of Declarative as well as the `DeferredReflection` class. Essentially, the extension provides a base class `AutomapBase` which automatically generates mapped classes and relationships between them based on given table metadata.

The `MetaData` in use normally might be produced via reflection, but there is no requirement that reflection is used. The most basic usage illustrates how `sqlalchemy.ext.automap` is able to deliver mapped classes, including relationships, based on a reflected schema:

    from sqlalchemy.ext.automap import automap_base
    from sqlalchemy.orm import Session
    from sqlalchemy import create_engine

    Base = automap_base()

    # engine, suppose it has two tables 'user' and 'address' set up
    engine = create_engine("sqlite:///mydatabase.db")

    # reflect the tables
    Base.prepare(engine, reflect=True)

    # mapped classes are now created with names matching that of the table
    # name.
    User = Base.classes.user
    Address = Base.classes.address

    session = Session(engine)

    # rudimentary relationships are produced
    session.add(Address(email_address="foo@bar.com", user=User(name="foo")))
    session.commit()

    # collection-based relationships are by default named "<classname>_collection"
    print (u1.address_collection)

Beyond that, the `AutomapBase` class is a declarative base, and supports all the features that declarative does. The “automapping” feature can be used with an existing, explicitly declared schema to generate relationships and missing classes only. Naming schemes and relationship-production routines can be dropped in using callable functions.

It is hoped that the `AutomapBase` system provides a quick and modernized solution to the problem that the very famous SQLSoup also tries to solve, that of generating a quick and rudimentary object model from an existing database on the fly. By addressing the issue strictly at the mapper configuration level, and integrating fully with existing Declarative class techniques, `AutomapBase` seeks to provide a well-integrated approach to the issue of expediently auto-generating ad-hoc mappings.

See also

Automap

## Behavioral Improvements

Improvements that should produce no compatibility issues except in exceedingly rare and unusual hypothetical cases, but are good to be aware of in case there are unexpected issues.

### Many JOIN and LEFT OUTER JOIN expressions will no longer be wrapped in (SELECT * FROM ..) AS ANON_1

For many years, the SQLAlchemy ORM has been held back from being able to nest a JOIN inside the right side of an existing JOIN (typically a LEFT OUTER JOIN, as INNER JOINs could always be flattened):

    SELECT a.*, b.*, c.* FROM a LEFT OUTER JOIN (b JOIN c ON b.id = c.id) ON a.id

This was due to the fact that SQLite, even today, cannot parse a statement of the above format:

    SQLite version 3.7.15.2 2013-01-09 11:53:05
    Enter ".help" for instructions
    Enter SQL statements terminated with a ";"
    sqlite> create table a(id integer);
    sqlite> create table b(id integer);
    sqlite> create table c(id integer);
    sqlite> select a.id, b.id, c.id from a left outer join (b join c on b.id=c.id) on b.id=a.id;
    Error: no such column: b.id

Right-outer-joins are of course another way to work around right-side parenthesization; this would be significantly complicated and visually unpleasant to implement, but fortunately SQLite doesn’t support RIGHT OUTER JOIN either :):

    sqlite> select a.id, b.id, c.id from b join c on b.id=c.id
       ...> right outer join a on b.id=a.id;
    Error: RIGHT and FULL OUTER JOINs are not currently supported

Back in 2005, it wasn’t clear if other databases had trouble with this form, but today it seems clear every database tested except SQLite now supports it (Oracle 8, a very old database, doesn’t support the JOIN keyword at all, but SQLAlchemy has always had a simple rewriting scheme in place for Oracle’s syntax). To make matters worse, SQLAlchemy’s usual workaround of applying a SELECT often degrades performance on platforms like Postgresql and MySQL:

    SELECT a.*, anon_1.* FROM a LEFT OUTER JOIN (
                    SELECT b.id AS b_id, c.id AS c_id
                    FROM b JOIN c ON b.id = c.id
                ) AS anon_1 ON a.id=anon_1.b_id

A JOIN like the above form is commonplace when working with joined-table inheritance structures; any time `Query.join()` is used to join from some parent to a joined-table subclass, or when `joinedload()` is used similarly, SQLAlchemy’s ORM would always make sure a nested JOIN was never rendered, lest the query wouldn’t be able to run on SQLite. Even though the Core has always supported a JOIN of the more compact form, the ORM had to avoid it.

An additional issue would arise when producing joins across many-to-many relationships where special criteria is present in the ON clause. Consider an eager load join like the following:

    session.query(Order).outerjoin(Order.items)

Assuming a many-to-many from `Order` to `Item` which actually refers to a subclass like `Subitem`, the SQL for the above would look like:

    SELECT order.id, order.name
    FROM order LEFT OUTER JOIN order_item ON order.id = order_item.order_id
    LEFT OUTER JOIN item ON order_item.item_id = item.id AND item.type = 'subitem'

What’s wrong with the above query? Basically, that it will load many `order` / `order_item` rows where the criteria of `item.type == 'subitem'` is not true.

As of SQLAlchemy 0.9, an entirely new approach has been taken. The ORM no longer worries about nesting JOINs in the right side of an enclosing JOIN, and it now will render these as often as possible while still returning the correct results. When the SQL statement is passed to be compiled, the dialect compiler will rewrite the join to suit the target backend, if that backend is known to not support a right-nested JOIN (which currently is only SQLite - if other backends have this issue please let us know!).

So a regular `query(Parent).join(Subclass)` will now usually produce a simpler expression:

    SELECT parent.id AS parent_id
    FROM parent JOIN (
            base_table JOIN subclass_table
            ON base_table.id = subclass_table.id) ON parent.id = base_table.parent_id

Joined eager loads like `query(Parent).options(joinedload(Parent.subclasses))` will alias the individual tables instead of wrapping in an `ANON_1`:

    SELECT parent.*, base_table_1.*, subclass_table_1.* FROM parent
        LEFT OUTER JOIN (
            base_table AS base_table_1 JOIN subclass_table AS subclass_table_1
            ON base_table_1.id = subclass_table_1.id)
            ON parent.id = base_table_1.parent_id

Many-to-many joins and eagerloads will right nest the “secondary” and “right” tables:

    SELECT order.id, order.name
    FROM order LEFT OUTER JOIN
    (order_item JOIN item ON order_item.item_id = item.id AND item.type = 'subitem')
    ON order_item.order_id = order.id

All of these joins, when rendered with a `Select` statement that specifically specifies `use_labels=True`, which is true for all the queries the ORM emits, are candidates for “join rewriting”, which is the process of rewriting all those right-nested joins into nested SELECT statements, while maintaining the identical labeling used by the `Select`. So SQLite, the one database that won’t support this very common SQL syntax even in 2013, shoulders the extra complexity itself, with the above queries rewritten as:

    -- sqlite only!
    SELECT parent.id AS parent_id
        FROM parent JOIN (
            SELECT base_table.id AS base_table_id,
                    base_table.parent_id AS base_table_parent_id,
                    subclass_table.id AS subclass_table_id
            FROM base_table JOIN subclass_table ON base_table.id = subclass_table.id
        ) AS anon_1 ON parent.id = anon_1.base_table_parent_id

    -- sqlite only!
    SELECT parent.id AS parent_id, anon_1.subclass_table_1_id AS subclass_table_1_id,
            anon_1.base_table_1_id AS base_table_1_id,
            anon_1.base_table_1_parent_id AS base_table_1_parent_id
    FROM parent LEFT OUTER JOIN (
        SELECT base_table_1.id AS base_table_1_id,
            base_table_1.parent_id AS base_table_1_parent_id,
            subclass_table_1.id AS subclass_table_1_id
        FROM base_table AS base_table_1
        JOIN subclass_table AS subclass_table_1 ON base_table_1.id = subclass_table_1.id
    ) AS anon_1 ON parent.id = anon_1.base_table_1_parent_id

    -- sqlite only!
    SELECT "order".id AS order_id
    FROM "order" LEFT OUTER JOIN (
            SELECT order_item_1.order_id AS order_item_1_order_id,
                order_item_1.item_id AS order_item_1_item_id,
                item.id AS item_id, item.type AS item_type
    FROM order_item AS order_item_1
        JOIN item ON item.id = order_item_1.item_id AND item.type IN (?)
    ) AS anon_1 ON "order".id = anon_1.order_item_1_order_id

The `Join.alias()`, `aliased()` and `with_polymorphic()` functions now support a new argument, `flat=True`, which is used to construct aliases of joined-table entities without embedding into a SELECT. This flag is not on by default, to help with backwards compatibility - but now a “polymorhpic” selectable can be joined as a target without any subqueries generated:

    employee_alias = with_polymorphic(Person, [Engineer, Manager], flat=True)

    session.query(Company).join(
                        Company.employees.of_type(employee_alias)
                    ).filter(
                        or_(
                            Engineer.primary_language == 'python',
                            Manager.manager_name == 'dilbert'
                        )
                    )

Generates (everywhere except SQLite):

    SELECT companies.company_id AS companies_company_id, companies.name AS companies_name
    FROM companies JOIN (
        people AS people_1
        LEFT OUTER JOIN engineers AS engineers_1 ON people_1.person_id = engineers_1.person_id
        LEFT OUTER JOIN managers AS managers_1 ON people_1.person_id = managers_1.person_id
    ) ON companies.company_id = people_1.company_id
    WHERE engineers.primary_language = %(primary_language_1)s
        OR managers.manager_name = %(manager_name_1)s

#2369 #2587

### ORM can efficiently fetch just-generated INSERT/UPDATE defaults using RETURNING

The `Mapper` has long supported an undocumented flag known as `eager_defaults=True`. The effect of this flag is that when an INSERT or UPDATE proceeds, and the row is known to have server-generated default values, a SELECT would immediately follow it in order to “eagerly” load those new values. Normally, the server-generated columns are marked as “expired” on the object, so that no overhead is incurred unless the application actually accesses these columns soon after the flush. The `eager_defaults` flag was therefore not of much use as it could only decrease performance, and was present only to support exotic event schemes where users needed default values to be available immediately within the flush process.

In 0.9, as a result of the version id enhancements, `eager_defaults` can now emit a RETURNING clause for these values, so on a backend with strong RETURNING support in particular Postgresql, the ORM can fetch newly generated default and SQL expression values inline with the INSERT or UPDATE. `eager_defaults`, when enabled, makes use of RETURNING automatically when the target backend and `Table` supports “implicit returning”.

### Subquery Eager Loading will apply DISTINCT to the innermost SELECT for some queries

In an effort to reduce the number of duplicate rows that can be generated by subquery eager loading when a many-to-one relationship is involved, a DISTINCT keyword will be applied to the innermost SELECT when the join is targeting columns that do not comprise the primary key, as in when loading along a many to one.

That is, when subquery loading on a many-to-one from A->B:

    SELECT b.id AS b_id, b.name AS b_name, anon_1.b_id AS a_b_id
    FROM (SELECT DISTINCT a_b_id FROM a) AS anon_1
    JOIN b ON b.id = anon_1.a_b_id

Since `a.b_id` is a non-distinct foreign key, DISTINCT is applied so that redundant `a.b_id` are eliminated. The behavior can be turned on or off unconditionally for a particular `relationship()` using the flag `distinct_target_key`, setting the value to `True` for unconditionally on, `False` for unconditionally off, and `None` for the feature to take effect when the target SELECT is against columns that do not comprise a full primary key. In 0.9, `None` is the default.

The option is also backported to 0.8 where the `distinct_target_key` option defaults to `False`.

While the feature here is designed to help performance by eliminating duplicate rows, the `DISTINCT` keyword in SQL itself can have a negative performance impact. If columns in the SELECT are not indexed, `DISTINCT` will likely perform an `ORDER BY` on the rowset which can be expensive. By keeping the feature limited just to foreign keys which are hopefully indexed in any case, it’s expected that the new defaults are reasonable.

The feature also does not eliminate every possible dupe-row scenario; if a many-to-one is present elsewhere in the chain of joins, dupe rows may still be present.

#2836

### Backref handlers can now propagate more than one level deep

The mechanism by which attribute events pass along their “initiator”, that is the object associated with the start of the event, has been changed; instead of a `AttributeImpl` being passed, a new object `attributes.Event` is passed instead; this object refers to the `AttributeImpl` as well as to an “operation token”, representing if the operation is an append, remove, or replace operation.

The attribute event system no longer looks at this “initiator” object in order to halt a recursive series of attribute events. Instead, the system of preventing endless recursion due to mutually-dependent backref handlers has been moved to the ORM backref event handlers specifically, which now take over the role of ensuring that a chain of mutually-dependent events (such as append to collection A.bs, set many-to-one attribute B.a in response) doesn’t go into an endless recursion stream. The rationale here is that the backref system, given more detail and control over event propagation, can finally allow operations more than one level deep to occur; the typical scenario is when a collection append results in a many-to-one replacement operation, which in turn should cause the item to be removed from a previous collection:

    class Parent(Base):
        __tablename__ = 'parent'

        id = Column(Integer, primary_key=True)
        children = relationship("Child", backref="parent")

    class Child(Base):
        __tablename__ = 'child'

        id = Column(Integer, primary_key=True)
        parent_id = Column(ForeignKey('parent.id'))

    p1 = Parent()
    p2 = Parent()
    c1 = Child()

    p1.children.append(c1)

    assert c1.parent is p1  # backref event establishes c1.parent as p1

    p2.children.append(c1)

    assert c1.parent is p2  # backref event establishes c1.parent as p2
    assert c1 not in p1.children  # second backref event removes c1 from p1.children

Above, prior to this change, the `c1` object would still have been present in `p1.children`, even though it is also present in `p2.children` at the same time; the backref handlers would have stopped at replacing `c1.parent` with `p2` instead of `p1`. In 0.9, using the more detailed `Event` object as well as letting the backref handlers make more detailed decisions about these objects, the propagation can continue onto removing `c1` from `p1.children` while maintaining a check against the propagation from going into an endless recursive loop.

End-user code which a. makes use of the `AttributeEvents.set()`, `AttributeEvents.append()`, or `AttributeEvents.remove()` events, and b. initiates further attribute modification operations as a result of these events may need to be modified to prevent recursive loops, as the attribute system no longer stops a chain of events from propagating endlessly in the absense of the backref event handlers. Additionally, code which depends upon the value of the `initiator` will need to be adjusted to the new API, and furthermore must be ready for the value of `initiator` to change from its original value within a string of backref-initiated events, as the backref handlers may now swap in a new `initiator` value for some operations.

#2789

### The typing system now handles the task of rendering “literal bind” values

A new method is added to `TypeEngine` `TypeEngine.literal_processor()` as well as `TypeDecorator.process_literal_param()` for `TypeDecorator` which take on the task of rendering so-called “inline literal paramters” - parameters that normally render as “bound” values, but are instead being rendered inline into the SQL statement due to the compiler configuration. This feature is used when generating DDL for constructs such as `CheckConstraint`, as well as by Alembic when using constructs such as `op.inline_literal()`. Previously, a simple “isinstance” check checked for a few basic types, and the “bind processor” was used unconditionally, leading to such issues as strings being encoded into utf-8 prematurely.

Custom types written with `TypeDecorator` should continue to work in “inline literal” scenarios, as the `TypeDecorator.process_literal_param()` falls back to `TypeDecorator.process_bind_param()` by default, as these methods usually handle a data manipulation, not as much how the data is presented to the database. `TypeDecorator.process_literal_param()` can be specified to specifically produce a string representing how a value should be rendered into an inline DDL statement.

#2838

### Schema identifiers now carry along their own quoting information

This change simplifies the Core’s usage of so-called “quote” flags, such as the `quote` flag passed to `Table` and `Column`. The flag is now internalized within the string name itself, which is now represented as an instance of `quoted_name`, a string subclass. The `IdentifierPreparer` now relies solely on the quoting preferences reported by the `quoted_name` object rather than checking for any explicit `quote` flags in most cases. The issue resolved here includes that various case-sensitive methods such as `Engine.has_table()` as well as similar methods within dialects now function with explicitly quoted names, without the need to complicate or introduce backwards-incompatible changes to those APIs (many of which are 3rd party) with the details of quoting flags - in particular, a wider range of identifiers now function correctly with the so-called “uppercase” backends like Oracle, Firebird, and DB2 (backends that store and report upon table and column names using all uppercase for case insensitive names).

The `quoted_name` object is used internally as needed; however if other keywords require fixed quoting preferences, the class is available publically.

#2812

### Improved rendering of Boolean constants, NULL constants, conjunctions

New capabilities have been added to the `true()` and `false()` constants, in particular in conjunction with `and_()` and `or_()` functions as well as the behavior of the WHERE/HAVING clauses in conjunction with these types, boolean types overall, and the `null()` constant.

Starting with a table such as this:

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

    t1 = Table('t', MetaData(), Column('x', Boolean()), Column('y', Integer))

A select construct will now render the boolean column as a binary expression on backends that don’t feature `true`/`false` constant beahvior:

    >>> from sqlalchemy import select, and_, false, true
    >>> from sqlalchemy.dialects import mysql, postgresql

    >>> print select([t1]).where(t1.c.x).compile(dialect=mysql.dialect())
    SELECT t.x, t.y  FROM t WHERE t.x = 1

The `and_()` and `or_()` constructs will now exhibit quasi “short circuit” behavior, that is truncating a rendered expression, when a `true()` or `false()` constant is present:

    >>> print select([t1]).where(and_(t1.c.y > 5, false())).compile(
    ...     dialect=postgresql.dialect())
    SELECT t.x, t.y FROM t WHERE false

`true()` can be used as the base to build up an expression:

    >>> expr = true()
    >>> expr = expr & (t1.c.y > 5)
    >>> print select([t1]).where(expr)
    SELECT t.x, t.y FROM t WHERE t.y > :y_1

The boolean constants `true()` and `false()` themselves render as `0 = 1` and `1 = 1` for a backend with no boolean constants:

    >>> print select([t1]).where(and_(t1.c.y > 5, false())).compile(
    ...     dialect=mysql.dialect())
    SELECT t.x, t.y FROM t WHERE 0 = 1

Interpretation of `None`, while not particularly valid SQL, is at least now consistent:

    >>> print select([t1.c.x]).where(None)
    SELECT t.x FROM t WHERE NULL

    >>> print select([t1.c.x]).where(None).where(None)
    SELECT t.x FROM t WHERE NULL AND NULL

    >>> print select([t1.c.x]).where(and_(None, None))
    SELECT t.x FROM t WHERE NULL AND NULL

#2804

### Label constructs can now render as their name alone in an ORDER BY

For the case where a `Label` is used in both the columns clause as well as the ORDER BY clause of a SELECT, the label will render as just it’s name in the ORDER BY clause, assuming the underlying dialect reports support of this feature.

E.g. an example like:

    from sqlalchemy.sql import table, column, select, func

    t = table('t', column('c1'), column('c2'))
    expr = (func.foo(t.c.c1) + t.c.c2).label("expr")

    stmt = select([expr]).order_by(expr)

    print stmt

Prior to 0.9 would render as:

    SELECT foo(t.c1) + t.c2 AS expr
    FROM t ORDER BY foo(t.c1) + t.c2

And now renders as:

    SELECT foo(t.c1) + t.c2 AS expr
    FROM t ORDER BY expr

The ORDER BY only renders the label if the label isn’t further embedded into an expression within the ORDER BY, other than a simple `ASC` or `DESC`.

The above format works on all databases tested, but might have compatibility issues with older database versions (MySQL 4? Oracle 8? etc.). Based on user reports we can add rules that will disable the feature based on database version detection.

#1068

### `RowProxy` now has tuple-sorting behavior

The `RowProxy` object acts much like a tuple, but up until now would not sort as a tuple if a list of them were sorted using `sorted()`. The `__eq__()` method now compares both sides as a tuple and also an `__lt__()` method has been added:

    users.insert().execute(
            dict(user_id=1, user_name='foo'),
            dict(user_id=2, user_name='bar'),
            dict(user_id=3, user_name='def'),
        )

    rows = users.select().order_by(users.c.user_name).execute().fetchall()

    eq_(rows, [(2, 'bar'), (3, 'def'), (1, 'foo')])

    eq_(sorted(rows), [(1, 'foo'), (2, 'bar'), (3, 'def')])

#2848

### A bindparam() construct with no type gets upgraded via copy when a type is available

The logic which “upgrades” a `bindparam()` construct to take on the type of the enclosing expression has been improved in two ways. First, the `bindparam()` object is copied before the new type is assigned, so that the given `bindparam()` is not mutated in place. Secondly, this same operation occurs when an `Insert` or `Update` construct is compiled, regarding the “values” that were set in the statement via the `ValuesBase.values()` method.

If given an untyped `bindparam()`:

    bp = bindparam("some_col")

If we use this parameter as follows:

    expr = mytable.c.col == bp

The type for `bp` remains as `NullType`, however if `mytable.c.col` is of type `String`, then `expr.right`, that is the right side of the binary expression, will take on the `String` type. Previously, `bp` itself would have been changed in place to have `String` as its type.

Similarly, this operation occurs in an `Insert` or `Update`:

    stmt = mytable.update().values(col=bp)

Above, `bp` remains unchanged, but the `String` type will be used when the statement is executed, which we can see by examining the `binds` dictionary:

    >>> compiled = stmt.compile()
    >>> compiled.binds['some_col'].type
    String

The feature allows custom types to take their expected effect within INSERT/UPDATE statements without needing to explicitly specify those types within every `bindparam()` expression.

The potentially backwards-compatible changes involve two unlikely scenarios. Since the the bound parameter is cloned, users should not be relying upon making in-place changes to a `bindparam()` construct once created. Additionally, code which uses `bindparam()` within an `Insert` or `Update` statement which is relying on the fact that the `bindparam()` is not typed according to the column being assigned towards will no longer function in that way.

#2850

### Columns can reliably get their type from a column referred to via ForeignKey

There’s a long standing behavior which says that a `Column` can be declared without a type, as long as that `Column` is referred to by a `ForeignKeyConstraint`, and the type from the referenced column will be copied into this one. The problem has been that this feature never worked very well and wasn’t maintained. The core issue was that the `ForeignKey` object doesn’t know what target `Column` it refers to until it is asked, typically the first time the foreign key is used to construct a `Join`. So until that time, the parent `Column` would not have a type, or more specifically, it would have a default type of `NullType`.

While it’s taken a long time, the work to reorganize the initialization of `ForeignKey` objects has been completed such that this feature can finally work acceptably. At the core of the change is that the `ForeignKey.column` attribute no longer lazily initializes the location of the target `Column`; the issue with this system was that the owning `Column` would be stuck with `NullType` as its type until the `ForeignKey` happened to be used.

In the new version, the `ForeignKey` coordinates with the eventual `Column` it will refer to using internal attachment events, so that the moment the referencing `Column` is associated with the `MetaData`, all `ForeignKey` objects that refer to it will be sent a message that they need to initialize their parent column. This system is more complicated but works more solidly; as a bonus, there are now tests in place for a wide variety of `Column` / `ForeignKey` configuration scenarios and error messages have been improved to be very specific to no less than seven different error conditions.

Scenarios which now work correctly include:

1. The type on a `Column` is immediately present as soon as the target `Column` becomes associated with the same `MetaData`; this works no matter which side is configured first:

       >>> from sqlalchemy import Table, MetaData, Column, Integer, ForeignKey
       >>> metadata = MetaData()
       >>> t2 = Table('t2', metadata, Column('t1id', ForeignKey('t1.id')))
       >>> t2.c.t1id.type
       NullType()
       >>> t1 = Table('t1', metadata, Column('id', Integer, primary_key=True))
       >>> t2.c.t1id.type
       Integer()

2. The system now works with `ForeignKeyConstraint` as well:

       >>> from sqlalchemy import Table, MetaData, Column, Integer, ForeignKeyConstraint
       >>> metadata = MetaData()
       >>> t2 = Table('t2', metadata,
       ...     Column('t1a'), Column('t1b'),
       ...     ForeignKeyConstraint(['t1a', 't1b'], ['t1.a', 't1.b']))
       >>> t2.c.t1a.type
       NullType()
       >>> t2.c.t1b.type
       NullType()
       >>> t1 = Table('t1', metadata,
       ...     Column('a', Integer, primary_key=True),
       ...     Column('b', Integer, primary_key=True))
       >>> t2.c.t1a.type
       Integer()
       >>> t2.c.t1b.type
       Integer()

3. It even works for “multiple hops” - that is, a `ForeignKey` that refers to a `Column` that refers to another `Column`:

       >>> from sqlalchemy import Table, MetaData, Column, Integer, ForeignKey
       >>> metadata = MetaData()
       >>> t2 = Table('t2', metadata, Column('t1id', ForeignKey('t1.id')))
       >>> t3 = Table('t3', metadata, Column('t2t1id', ForeignKey('t2.t1id')))
       >>> t2.c.t1id.type
       NullType()
       >>> t3.c.t2t1id.type
       NullType()
       >>> t1 = Table('t1', metadata, Column('id', Integer, primary_key=True))
       >>> t2.c.t1id.type
       Integer()
       >>> t3.c.t2t1id.type
       Integer()

#1765

## Dialect Changes

### Firebird `fdb` is now the default Firebird dialect.

The `fdb` dialect is now used if an engine is created without a dialect specifier, i.e. `firebird://`. `fdb` is a `kinterbasdb` compatible DBAPI which per the Firebird project is now their official Python driver.

#2504

### Firebird `fdb` and `kinterbasdb` set `retaining=False` by default

Both the `fdb` and `kinterbasdb` DBAPIs support a flag `retaining=True` which can be passed to the `commit()` and `rollback()` methods of its connection. The documented rationale for this flag is so that the DBAPI can re-use internal transaction state for subsequent transactions, for the purposes of improving performance. However, newer documentation refers to analyses of Firebird’s “garbage collection” which expresses that this flag can have a negative effect on the database’s ability to process cleanup tasks, and has been reported as lowering performance as a result.

It’s not clear how this flag is actually usable given this information, and as it appears to be only a performance enhancing feature, it now defaults to `False`. The value can be controlled by passing the flag `retaining=True` to the `create_engine()` call. This is a new flag which is added as of 0.8.2, so applications on 0.8.2 can begin setting this to `True` or `False` as desired.

See also

`sqlalchemy.dialects.firebird.fdb`

`sqlalchemy.dialects.firebird.kinterbasdb`

http://pythonhosted.org/fdb/usage-guide.html#retaining-transactions - information on the “retaining” flag.

#2763