Release: 1.3.0b1 pre release | Release Date: unreleased

SQLAlchemy 1.3 Documentation

SQLAlchemy 1.3 Documentation

Changes and Migration

Project Versions

What’s New in SQLAlchemy 1.3?

About this Document

This document describes changes between SQLAlchemy version 1.2 and SQLAlchemy version 1.3.


This guide introduces what’s new in SQLAlchemy version 1.3 and also documents changes which affect users migrating their applications from the 1.2 series of SQLAlchemy to 1.3.

Please carefully review the sections on behavioral changes for potentially backwards-incompatible changes in behavior.

New Features and Improvements - ORM

selectin loading no longer uses JOIN for simple one-to-many

The “selectin” loading feature added in 1.2 introduced an extremely performant new way to eagerly load collections, in many cases much faster than that of “subquery” eager loading, as it does not rely upon restating the original SELECT query and instead uses a simple IN clause. However, the “selectin” load still relied upon rendering a JOIN between the parent and related tables, since it needs the parent primary key values in the row in order to match rows up. In 1.3, a new optimization is added which will omit this JOIN in the most common case of a simple one-to-many load, where the related row already contains the primary key of the parent row expressed in its foreign key columns. This again provides for a dramatic performance improvement as the ORM now can load large numbers of collections all in one query without using JOIN or subqueries at all.

Given a mapping:

class A(Base):
    __tablename__ = 'a'

    id = Column(Integer, primary_key=True)
    bs = relationship("B", lazy="selectin")

class B(Base):
    __tablename__ = 'b'
    id = Column(Integer, primary_key=True)
    a_id = Column(ForeignKey(""))

In the 1.2 version of “selectin” loading, a load of A to B looks like:

SELECT AS a_1_id, AS b_id, b.a_id AS b_a_id
FROM a AS a_1 JOIN b ON = b.a_id
WHERE IN (?, ?, ?, ?, ?, ?, ?, ?, ?, ?) ORDER BY
(1, 2, 3, 4, 5, 6, 7, 8, 9, 10)

With the new behavior, the load looks like:

SELECT b.a_id AS b_a_id, AS b_id FROM b
WHERE b.a_id IN (?, ?, ?, ?, ?, ?, ?, ?, ?, ?) ORDER BY b.a_id
(1, 2, 3, 4, 5, 6, 7, 8, 9, 10)

The behavior is being released as automatic, using a similar heuristic that lazy loading uses in order to determine if related entities can be fetched directly from the identity map. However, as with most querying features, the feature’s implementation became more complex as a result of advanced scenarios regarding polymorphic loading. If problems are encountered, users should report a bug, however the change also incldues a flag relationship.omit_join which can be set to False on the relationship() to disable the optimization.


info dictionary added to InstanceState

Added the .info dictionary to the InstanceState class, the object that comes from calling inspect() on a mapped object. This allows custom recipes to add additional information about an object that will be carried along with that object’s full lifecycle in memory:

from sqlalchemy import inspect

u1 = User(id=7, name='ed')

inspect(u1).info['user_info'] = '7|ed'


Horizontal Sharding extension supports bulk update and delete methods

The ShardedQuery extension object supports the Query.update() and Query.delete() bulk update/delete methods. The query_chooser callable is consulted when they are called in order to run the update/delete across multiple shards based on given criteria.


Key Behavioral Changes - ORM

Association proxy has new cascade_scalar_deletes flag

Given a mapping as:

class A(Base):
    __tablename__ = 'test_a'
    id = Column(Integer, primary_key=True)
    ab = relationship(
        'AB', backref='a', uselist=False)
    b = association_proxy(
        'ab', 'b', creator=lambda b: AB(b=b),

class B(Base):
    __tablename__ = 'test_b'
    id = Column(Integer, primary_key=True)
    ab = relationship('AB', backref='b', cascade='all, delete-orphan')

class AB(Base):
    __tablename__ = 'test_ab'
    a_id = Column(Integer, ForeignKey(, primary_key=True)
    b_id = Column(Integer, ForeignKey(, primary_key=True)

An assigment to A.b will generate an AB object:

a.b = B()

The A.b association is scalar, and includes a new flag AssociationProxy.cascade_scalar_deletes. When set, setting A.b to None will remove A.ab as well. The default behavior remains that it leaves a.ab in place:

a.b = None
assert a.ab is None

While it at first seemed intuitive that this logic should just look at the “cascade” attribute of the existing relationship, it’s not clear from that alone if the proxied object should be removed, hence the behavior is made available as an explicit option.

Additionally, del now works for scalars in a similar manner as setting to None:

del a.b
assert a.ab is None


AssociationProxy stores class-specific state in a separate container

The AssociationProxy object makes lots of decisions based on the parent mapped class it is associated with. While the AssociationProxy historically began as a relatively simple “getter”, it became apparent early on that it also needed to make decisions about what kind of attribute it is referring towards, e.g. scalar or collection, mapped object or simple value, and similar. To achieve this, it needs to inspect the mapped attribute or other descriptor or attribute that it refers towards, as referenced from its parent class. However in Python descriptor mechanics, a descriptor only learns about its “parent” class when it is accessed in the context of that class, such as calling MyClass.some_descriptor, which calls the __get__() method which passes in the class. The AssociationProxy object would therefore store state that is specific to that class, but only once this method were called; trying to inspect this state ahead of time without first accessing the AssociationProxy as a descriptor would raise an error. Additionally, it would assume that the first class to be seen by __get__() would be the only parent class it needed to know about. This is despite the fact that if a particular class has inheriting subclasses, the association proxy is really working on behalf of more than one parent class even though it was not explicitly re-used. While even with this shortcoming, the association proxy would still get pretty far with its current behavior, it still leaves shortcomings in some cases as well as the complex problem of determining the best “owner” class.

These problems are now solved in that AssociationProxy no longer modifies its own internal state when __get__() is called; instead, a new object is generated per-class known as AssociationProxyInstance which handles all the state specific to a particular mapped parent class (when the parent class is not mapped, no AssociationProxyInstance is generated). The concept of a single “owning class” for the association proxy, which was nonetheless improved in 1.1, has essentially been replaced with an approach where the AP now can treat any number of “owning” classes equally.

To accommodate for applications that want to inspect this state for an AssociationProxy without necessarily calling __get__(), a new method AssociationProxy.for_class() is added that provides direct access to a class-specific AssociationProxyInstance, demonstrated as:

class User(Base):
    # ...

    keywords = association_proxy('kws', 'keyword')

proxy_state = inspect(User).all_orm_descriptors["keywords"].for_class(User)

Once we have the AssociationProxyInstance object, in the above example stored in the proxy_state variable, we can look at attributes specific to the User.keywords proxy, such as target_class:

>>> proxy_state.target_class


FOR UPDATE clause is rendered within the joined eager load subquery as well as outside

This change applies specifically to the use of the joinedload() loading strategy in conjunction with a row limited query, e.g. using Query.first() or Query.limit(), as well as with use of the Query.with_for_update method.

Given a query as:


The Query object renders a SELECT of the following form when joined eager loading is combined with LIMIT:

SELECT subq.a_id, subq.a_data,, FROM (
    SELECT AS a_id, AS a_data FROM a LIMIT 5
) AS subq LEFT OUTER JOIN b ON subq.a_id=b.a_id

This is so that the limit of rows takes place for the primary entity without affecting the joined eager load of related items. When the above query is combined with “SELECT..FOR UPDATE”, the behavior has been this:

SELECT subq.a_id, subq.a_data,, FROM (
    SELECT AS a_id, AS a_data FROM a LIMIT 5
) AS subq LEFT OUTER JOIN b ON subq.a_id=b.a_id FOR UPDATE

However, MySQL due to does not lock the rows inside the subquery, unlike that of Postgresql and other databases. So the above query now renders as:

SELECT subq.a_id, subq.a_data,, FROM (
) AS subq LEFT OUTER JOIN b ON subq.a_id=b.a_id FOR UPDATE

On the Oracle dialect, the inner “FOR UPDATE” is not rendered as Oracle does not support this syntax and the dialect skips any “FOR UPDATE” that is against a subquery; it isn’t necessary in any case since Oracle, like Postgresql, correctly locks all elements of the returned row.

When using the Query.with_for_update.of modifier, typically on Postgresql, the outer “FOR UPDATE” is omitted, and the OF is now rendered on the inside; previously, the OF target would not be converted to accommodate for the subquery correctly. So given:


The query would now render as:

SELECT subq.a_id, subq.a_data,, FROM (
    SELECT AS a_id, AS a_data FROM a LIMIT 5 FOR UPDATE OF a
) AS subq LEFT OUTER JOIN b ON subq.a_id=b.a_id

The above form should be helpful on Postgresql additionally since Postgresql will not allow the FOR UPDATE clause to be rendered after the LEFT OUTER JOIN target.

Overall, FOR UPDATE remains highly specific to the target database in use and can’t easily be generalized for more complex queries.


passive_deletes=’all’ will leave FK unchanged for object removed from collection

The relationship.passive_deletes option accepts the value "all" to indicate that no foreign key attributes should be modified when the object is flushed, even if the relationship’s collection / reference has been removed. Previously, this did not take place for one-to-many, or one-to-one relationships, in the following situation:

class User(Base):
    __tablename__ = 'users'

    id = Column(Integer, primary_key=True)
    addresses = relationship(

class Address(Base):
    __tablename__ = 'addresses'
    id = Column(Integer, primary_key=True)
    email = Column(String)

    user_id = Column(Integer, ForeignKey(''))
    user = relationship("User")

u1 = session.query(User).first()
address = u1.addresses[0]

# would fail and be set to None
assert address.user_id ==

The fix now includes that address.user_id is left unchanged as per passive_deletes="all". This kind of thing is useful for building custom “version table” schemes and such where rows are archived instead of deleted.


Association Proxy now Strong References the Parent Object

The long-standing behavior of the association proxy collection maintaining only a weak reference to the parent object is reverted; the proxy will now maintain a strong reference to the parent for as long as the proxy collection itself is also in memory, eliminating the “stale association proxy” error. This change is being made on an experimental basis to see if any use cases arise where it causes side effects.

As an example, given a mapping with association proxy:

class A(Base):
    __tablename__ = 'a'

    id = Column(Integer, primary_key=True)
    bs = relationship("B")
    b_data = association_proxy('bs', 'data')

class B(Base):
    __tablename__ = 'b'
    id = Column(Integer, primary_key=True)
    a_id = Column(ForeignKey(""))
    data = Column(String)

a1 = A(bs=[B(data='b1'), B(data='b2')])

b_data = a1.b_data

Previously, if a1 were deleted out of scope:

del a1

Trying to iterate the b_data collection after a1 is deleted from scope would raise the error "stale association proxy, parent object has gone out of scope". This is because the association proxy needs to access the actual collection in order to produce a view, and prior to this change it maintained only a weak reference to a1. In particular, users would frequently encounter this error when performing an inline operation such as:

collection = session.query(A).filter_by(id=1).first().b_data

Above, because the A object would be garbage collected before the b_data collection were actually used.

The change is that the b_data collection is now maintaining a strong reference to the a1 object, so that it remains present:

assert b_data == ['b1', 'b2']

This change introduces the side effect that if an application is passing around the collection as above, the parent object won’t be garbage collected until the collection is also discarded. As always, if a1 is persistent inside a particular Session, it will remain part of that session’s state until it is garbage collected.

Note that this change may be revised if it leads to problems.


New Features and Improvements - Core

Binary comparison interpretation for SQL functions

This enhancement is implemented at the Core level, however is applicable primarily to the ORM.

A SQL function that compares two elements can now be used as a “comparison” object, suitable for usage in an ORM relationship(), by first creating the function as usual using the func factory, then when the function is complete calling upon the FunctionElement.as_comparison() modifier to produce a BinaryExpression that has a “left” and a “right” side:

class Venue(Base):
    __tablename__ = 'venue'
    id = Column(Integer, primary_key=True)
    name = Column(String)

    descendants = relationship(
            remote(foreign(name)), name + "/"
        ).as_comparison(1, 2) == 1,

Above, the relationship.primaryjoin of the “descendants” relationship will produce a “left” and a “right” expression based on the first and second arguments passed to instr(). This allows features like the ORM lazyload to produce SQL like:

SELECT AS venue_id, AS venue_name
FROM venue
WHERE instr(, (? || ?)) = ? ORDER BY
('parent1', '/', 1)

and a joinedload, such as:

v1 = s.query(Venue).filter_by(name="parent1").options(

to work as:

SELECT AS venue_id, AS venue_name, AS venue_1_id, AS venue_1_name
FROM venue LEFT OUTER JOIN venue AS venue_1
  ON instr(, ( || ?)) = ?
('/', 1, 'parent1')

This feature is expected to help with situations such as making use of geometric functions in relationship join conditions, or any case where the ON clause of the SQL join is expressed in terms of a SQL function.


Expanding IN feature now supports empty lists

The “expanding IN” feature introduced in version 1.2 at Late-expanded IN parameter sets allow IN expressions with cached statements now supports empty lists passed to the ColumnOperators.in_() operator. The implementation for an empty list will produce an “empty set” expression that is specific to a target backend, such as “SELECT CAST(NULL AS INTEGER) WHERE 1!=1” for Postgresql, “SELECT 1 FROM (SELECT 1) as _empty_set WHERE 1!=1” for MySQL:

>>> from sqlalchemy import create_engine
>>> from sqlalchemy import select, literal_column, bindparam
>>> e = create_engine("postgresql://scott:tiger@localhost/test", echo=True)
>>> with e.connect() as conn:
...      conn.execute(
...          select([literal_column('1')]).
...          where(literal_column('1').in_(bindparam('q', expanding=True))),
...          q=[]
...      )

The feature also works for tuple-oriented IN statements, where the “empty IN” expression will be expanded to support the elements given inside the tuple, such as on Postgresql:

>>> from sqlalchemy import create_engine
>>> from sqlalchemy import select, literal_column, tuple_, bindparam
>>> e = create_engine("postgresql://scott:tiger@localhost/test", echo=True)
>>> with e.connect() as conn:
...      conn.execute(
...          select([literal_column('1')]).
...          where(tuple_(50, "somestring").in_(bindparam('q', expanding=True))),
...          q=[]
...      )
SELECT 1 WHERE (%(param_1)s, %(param_2)s)


TypeEngine methods bind_expression, column_expression work with Variant, type-specific types

The TypeEngine.bind_expression() and TypeEngine.column_expression() methods now work when they are present on the “impl” of a particular datatype, allowing these methods to be used by dialects as well as for TypeDecorator and Variant use cases.

The following example illustrates a TypeDecorator that applies SQL-time conversion functions to a LargeBinary. In order for this type to work in the context of a Variant, the compiler needs to drill into the “impl” of the variant expression in order to locate these methods:

from sqlalchemy import TypeDecorator, LargeBinary, func

class CompressedLargeBinary(TypeDecorator):
    impl = LargeBinary

    def bind_expression(self, bindvalue):
        return func.compress(bindvalue, type_=self)

    def column_expression(self, col):
        return func.uncompress(col, type_=self)

MyLargeBinary = LargeBinary().with_variant(CompressedLargeBinary(), "sqlite")

The above expression will render a function within SQL when used on SQlite only:

from sqlalchemy import select, column
from sqlalchemy.dialects import sqlite
print(select([column('x', CompressedLargeBinary)]).compile(dialect=sqlite.dialect()))

will render:

SELECT uncompress(x) AS x

The change also includes that dialects can implement TypeEngine.bind_expression() and TypeEngine.column_expression() on dialect-level implementation types where they will now be used; in particular this will be used for MySQL’s new “binary prefix” requirement as well as for casting decimal bind values for MySQL.


New last-in-first-out strategy for QueuePool

The connection pool usually used by create_engine() is known as QueuePool. This pool uses an object equivalent to Python’s built-in Queue class in order to store database connections waiting to be used. The Queue features first-in-first-out behavior, which is intended to provide a round-robin use of the database connections that are persistently in the pool. However, a potential downside of this is that when the utilization of the pool is low, the re-use of each connection in series means that a server-side timeout strategy that attempts to reduce unused connections is prevented from shutting down these connections. To suit this use case, a new flag create_engine.pool_use_lifo is added which reverses the .get() method of the Queue to pull the connection from the beginning of the queue instead of the end, essentially turning the “queue” into a “stack” (adding a whole new pool called StackPool was considered, however this was too much verbosity).

Key Behavioral Changes - Core

Dialect Improvements and Changes - PostgreSQL

Added basic reflection support for Postgresql paritioned tables

SQLAlchemy can render the “PARTITION BY” sequnce within a Postgresql CREATE TABLE statement using the flag postgresql_partition_by, added in version 1.2.6. However, the 'p' type was not part of the reflection queries used until now.

Given a schema such as:

dv = Table(
    'data_values', metadata,
    Column('modulus', Integer, nullable=False),
    Column('data', String(30)),

        "CREATE TABLE data_values_4_10 PARTITION OF data_values "
        "FOR VALUES FROM (4) TO (10)")

The two table names 'data_values' and 'data_values_4_10' will come back from Inspector.get_table_names() and additionally the columns will come back from Inspector.get_columns('data_values') as well as Inspector.get_columns('data_values_4_10'). This also extends to the use of Table(..., autoload=True) with these tables.


Dialect Improvements and Changes - MySQL

Protocol-level ping now used for pre-ping

The MySQL dialects including mysqlclient, python-mysql, PyMySQL and mysql-connector-python now use the method for the pool pre-ping feature, described at Disconnect Handling - Pessimistic. This is a much more lightweight ping than the previous method of emitting “SELECT 1” on the connection.

Control of parameter ordering within ON DUPLICATE KEY UPDATE

The order of UPDATE parameters in the ON DUPLICATE KEY UPDATE clause can now be explcitly ordered by passing a list of 2-tuples:

from sqlalchemy.dialects.mysql import insert

insert_stmt = insert(my_table).values(
    data='inserted value')

on_duplicate_key_stmt = insert_stmt.on_duplicate_key_update(
        ("data", "some data"),
        ("updated_at", func.current_timestamp()),

Dialect Improvements and Changes - SQLite

Support for SQLite JSON Added

A new datatype sqlite.JSON is added which implements SQLite’s json member access functions on behalf of the types.JSON base datatype. The SQLite JSON_EXTRACT and JSON_QUOTE functions are used by the implementation to provide basic JSON support.

Note that the name of the datatype itself as rendered in the database is the name “JSON”. This will create a SQLite datatype with “numeric” affinity, which normally should not be an issue except in the case of a JSON value that consists of single integer value. Nevertheless, following an example in SQLite’s own documentation at the name JSON is being used for its familiarity.


Dialect Improvements and Changes - Oracle

National char datatypes de-emphasized for generic unicode, re-enabled with option

The Unicode and UnicodeText datatypes by default now correspond to the VARCHAR2 and CLOB datatypes on Oracle, rather than NVARCHAR2 and NCLOB (otherwise known as “national” character set types). This will be seen in behaviors such as that of how they render in CREATE TABLE statements, as well as that no type object will be passed to setinputsizes() when bound parameters using Unicode or UnicodeText are used; cx_Oracle handles the string value natively. This change is based on advice from cx_Oracle’s maintainer that the “national” datatypes in Oracle are largely obsolete and are not performant. They also interfere in some situations such as when applied to the format specifier for functions like trunc().

The one case where NVARCHAR2 and related types may be needed is for a database that is not using a Unicode-compliant character set. In this case, the flag use_nchar_for_unicode can be passed to create_engine() to re-enable the old behavior.

As always, using the oracle.NVARCHAR2 and oracle.NCLOB datatypes explicitly will continue to make use of NVARCHAR2 and NCLOB, including within DDL as well as when handling bound parameters with cx_Oracle’s setinputsizes().

On the read side, automatic Unicode conversion under Python 2 has been added to CHAR/VARCHAR/CLOB result rows, to match the behavior of cx_Oracle under Python 3. In order to mitigate the performance hit that the cx_Oracle dialect had previously with this behavior under Python 2, SQLAlchemy’s very performant (when C extensions are built) native Unicode handlers are used under Python 2. The automatic unicode coercion can be disabled by setting the coerce_to_unicode flag to False. This flag now defaults to True and applies to all string data returned in a result set that isn’t explicitly under Unicode or Oracle’s NVARCHAR2/NCHAR/NCLOB datatypes.


Dialect Improvements and Changes - SQL Server

Support for pyodbc fast_executemany

Pyodbc’s recently added “fast_executemany” mode, available when using the Microsoft ODBC driver, is now an option for the pyodbc / mssql dialect. Pass it via create_engine():

engine = create_engine(


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