Release: 2.0.44 | Release Date: October 10, 2025

# SQLAlchemy 2.0 Documentation

# What’s New in SQLAlchemy 0.6?

About this Document

This document describes changes between SQLAlchemy version 0.5, last released January 16, 2010, and SQLAlchemy version 0.6, last released May 5, 2012.

Document date: June 6, 2010

This guide documents API changes which affect users migrating their applications from the 0.5 series of SQLAlchemy to 0.6. Note that SQLAlchemy 0.6 removes some behaviors which were deprecated throughout the span of the 0.5 series, and also deprecates more behaviors specific to 0.5.

## Platform Support

- cPython versions 2.4 and upwards throughout the 2.xx series

- Jython 2.5.1 - using the zxJDBC DBAPI included with Jython.

- cPython 3.x - see [source:sqlalchemy/trunk/README.py3k] for information on how to build for python3.

## New Dialect System

Dialect modules are now broken up into distinct subcomponents, within the scope of a single database backend. Dialect implementations are now in the `sqlalchemy.dialects` package. The `sqlalchemy.databases` package still exists as a placeholder to provide some level of backwards compatibility for simple imports.

For each supported database, a sub-package exists within `sqlalchemy.dialects` where several files are contained. Each package contains a module called `base.py` which defines the specific SQL dialect used by that database. It also contains one or more “driver” modules, each one corresponding to a specific DBAPI - these files are named corresponding to the DBAPI itself, such as `pysqlite`, `cx_oracle`, or `pyodbc`. The classes used by SQLAlchemy dialects are first declared in the `base.py` module, defining all behavioral characteristics defined by the database. These include capability mappings, such as “supports sequences”, “supports returning”, etc., type definitions, and SQL compilation rules. Each “driver” module in turn provides subclasses of those classes as needed which override the default behavior to accommodate the additional features, behaviors, and quirks of that DBAPI. For DBAPIs that support multiple backends (pyodbc, zxJDBC, mxODBC), the dialect module will use mixins from the `sqlalchemy.connectors` package, which provide functionality common to that DBAPI across all backends, most typically dealing with connect arguments. This means that connecting using pyodbc, zxJDBC or mxODBC (when implemented) is extremely consistent across supported backends.

The URL format used by `create_engine()` has been enhanced to handle any number of DBAPIs for a particular backend, using a scheme that is inspired by that of JDBC. The previous format still works, and will select a “default” DBAPI implementation, such as the PostgreSQL URL below that will use psycopg2:

    create_engine("postgresql://scott:tiger@localhost/test")

However to specify a specific DBAPI backend such as pg8000, add it to the “protocol” section of the URL using a plus sign “+”:

    create_engine("postgresql+pg8000://scott:tiger@localhost/test")

Important Dialect Links:

- Documentation on connect arguments: https://www.sqlalchemy.org/docs/06/dbengine.html#create-engine-url-arguments.

- Reference documentation for individual dialects: https://www.sqlalchemy.org/docs/06/reference/dialects/index.html.

- The tips and tricks at DatabaseNotes.

Other notes regarding dialects:

- the type system has been changed dramatically in SQLAlchemy 0.6. This has an impact on all dialects regarding naming conventions, behaviors, and implementations. See the section on “Types” below.

- the `ResultProxy` object now offers a 2x speed improvement in some cases thanks to some refactorings.

- the `RowProxy`, i.e. individual result row object, is now directly pickleable.

- the setuptools entrypoint used to locate external dialects is now called `sqlalchemy.dialects`. An external dialect written against 0.4 or 0.5 will need to be modified to work with 0.6 in any case so this change does not add any additional difficulties.

- dialects now receive an initialize() event on initial connection to determine connection properties.

- Functions and operators generated by the compiler now use (almost) regular dispatch functions of the form “visit_<opname>” and “visit_<funcname>_fn” to provide customed processing. This replaces the need to copy the “functions” and “operators” dictionaries in compiler subclasses with straightforward visitor methods, and also allows compiler subclasses complete control over rendering, as the full _Function or _BinaryExpression object is passed in.

### Dialect Imports

The import structure of dialects has changed. Each dialect now exports its base “dialect” class as well as the full set of SQL types supported on that dialect via `sqlalchemy.dialects.<name>`. For example, to import a set of PG types:

    from sqlalchemy.dialects.postgresql import (
        INTEGER,
        BIGINT,
        SMALLINT,
        VARCHAR,
        MACADDR,
        DATE,
        BYTEA,
    )

Above, `INTEGER` is actually the plain `INTEGER` type from `sqlalchemy.types`, but the PG dialect makes it available in the same way as those types which are specific to PG, such as `BYTEA` and `MACADDR`.

## Expression Language Changes

### An Important Expression Language Gotcha

There’s one quite significant behavioral change to the expression language which may affect some applications. The boolean value of Python boolean expressions, i.e. `==`, `!=`, and similar, now evaluates accurately with regards to the two clause objects being compared.

As we know, comparing a `ClauseElement` to any other object returns another `ClauseElement`:

    >>> from sqlalchemy.sql import column
    >>> column("foo") == 5
    <sqlalchemy.sql.expression._BinaryExpression object at 0x1252490>

This so that Python expressions produce SQL expressions when converted to strings:

    >>> str(column("foo") == 5)
    'foo = :foo_1'

But what happens if we say this?

    >>> if column("foo") == 5:
    ...     print("yes")

In previous versions of SQLAlchemy, the returned `_BinaryExpression` was a plain Python object which evaluated to `True`. Now it evaluates to whether or not the actual `ClauseElement` should have the same hash value as to that being compared. Meaning:

    >>> bool(column("foo") == 5)
    False
    >>> bool(column("foo") == column("foo"))
    False
    >>> c = column("foo")
    >>> bool(c == c)
    True
    >>>

That means code such as the following:

    if expression:
        print("the expression is:", expression)

Would not evaluate if `expression` was a binary clause. Since the above pattern should never be used, the base `ClauseElement` now raises an exception if called in a boolean context:

    >>> bool(c)
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
      ...
        raise TypeError("Boolean value of this clause is not defined")
    TypeError: Boolean value of this clause is not defined

Code that wants to check for the presence of a `ClauseElement` expression should instead say:

    if expression is not None:
        print("the expression is:", expression)

Keep in mind, this applies to Table and Column objects too.

The rationale for the change is twofold:

- Comparisons of the form `if c1 == c2: <do something>` can actually be written now

- Support for correct hashing of `ClauseElement` objects now works on alternate platforms, namely Jython. Up until this point SQLAlchemy relied heavily on the specific behavior of cPython in this regard (and still had occasional problems with it).

### Stricter “executemany” Behavior

An “executemany” in SQLAlchemy corresponds to a call to `execute()`, passing along a collection of bind parameter sets:

    connection.execute(table.insert(), {"data": "row1"}, {"data": "row2"}, {"data": "row3"})

When the `Connection` object sends off the given `insert()` construct for compilation, it passes to the compiler the keynames present in the first set of binds passed along to determine the construction of the statement’s VALUES clause. Users familiar with this construct will know that additional keys present in the remaining dictionaries don’t have any impact. What’s different now is that all subsequent dictionaries need to include at least every key that is present in the first dictionary. This means that a call like this no longer works:

    connection.execute(
        table.insert(),
        {"timestamp": today, "data": "row1"},
        {"timestamp": today, "data": "row2"},
        {"data": "row3"},
    )

Because the third row does not specify the ‘timestamp’ column. Previous versions of SQLAlchemy would simply insert NULL for these missing columns. However, if the `timestamp` column in the above example contained a Python-side default value or function, it would not be used. This because the “executemany” operation is optimized for maximum performance across huge numbers of parameter sets, and does not attempt to evaluate Python-side defaults for those missing keys. Because defaults are often implemented either as SQL expressions which are embedded inline with the INSERT statement, or are server side expressions which again are triggered based on the structure of the INSERT string, which by definition cannot fire off conditionally based on each parameter set, it would be inconsistent for Python side defaults to behave differently vs. SQL/server side defaults. (SQL expression based defaults are embedded inline as of the 0.5 series, again to minimize the impact of huge numbers of parameter sets).

SQLAlchemy 0.6 therefore establishes predictable consistency by forbidding any subsequent parameter sets from leaving any fields blank. That way, there’s no more silent failure of Python side default values and functions, which additionally are allowed to remain consistent in their behavior versus SQL and server side defaults.

### UNION and other “compound” constructs parenthesize consistently

A rule that was designed to help SQLite has been removed, that of the first compound element within another compound (such as, a `union()` inside of an `except_()`) wouldn’t be parenthesized. This is inconsistent and produces the wrong results on PostgreSQL, which has precedence rules regarding INTERSECTION, and its generally a surprise. When using complex composites with SQLite, you now need to turn the first element into a subquery (which is also compatible on PG). A new example is in the SQL expression tutorial at the end of [https://www.sqlalchemy.org/docs/06/sqlexpression.html #unions-and-other-set-operations]. See #1665 and r6690 for more background.

## C Extensions for Result Fetching

The `ResultProxy` and related elements, including most common “row processing” functions such as unicode conversion, numerical/boolean conversions and date parsing, have been re-implemented as optional C extensions for the purposes of performance. This represents the beginning of SQLAlchemy’s path to the “dark side” where we hope to continue improving performance by reimplementing critical sections in C. The extensions can be built by specifying `--with-cextensions`, i.e. `python setup.py --with- cextensions install`.

The extensions have the most dramatic impact on result fetching using direct `ResultProxy` access, i.e. that which is returned by `engine.execute()`, `connection.execute()`, or `session.execute()`. Within results returned by an ORM `Query` object, result fetching is not as high a percentage of overhead, so ORM performance improves more modestly, and mostly in the realm of fetching large result sets. The performance improvements highly depend on the dbapi in use and on the syntax used to access the columns of each row (eg `row['name']` is much faster than `row.name`). The current extensions have no impact on the speed of inserts/updates/deletes, nor do they improve the latency of SQL execution, that is, an application that spends most of its time executing many statements with very small result sets will not see much improvement.

Performance has been improved in 0.6 versus 0.5 regardless of the extensions. A quick overview of what connecting and fetching 50,000 rows looks like with SQLite, using mostly direct SQLite access, a `ResultProxy`, and a simple mapped ORM object:

    sqlite select/native: 0.260s

    0.6 / C extension

    sqlalchemy.sql select: 0.360s
    sqlalchemy.orm fetch: 2.500s

    0.6 / Pure Python

    sqlalchemy.sql select: 0.600s
    sqlalchemy.orm fetch: 3.000s

    0.5 / Pure Python

    sqlalchemy.sql select: 0.790s
    sqlalchemy.orm fetch: 4.030s

Above, the ORM fetches the rows 33% faster than 0.5 due to in-python performance enhancements. With the C extensions we get another 20%. However, `ResultProxy` fetches improve by 67% with the C extension versus not. Other tests report as much as a 200% speed improvement for some scenarios, such as those where lots of string conversions are occurring.

## New Schema Capabilities

The `sqlalchemy.schema` package has received some long- needed attention. The most visible change is the newly expanded DDL system. In SQLAlchemy, it was possible since version 0.5 to create custom DDL strings and associate them with tables or metadata objects:

    from sqlalchemy.schema import DDL

    DDL("CREATE TRIGGER users_trigger ...").execute_at("after-create", metadata)

Now the full suite of DDL constructs are available under the same system, including those for CREATE TABLE, ADD CONSTRAINT, etc.:

    from sqlalchemy.schema import Constraint, AddConstraint

    AddContraint(CheckConstraint("value > 5")).execute_at("after-create", mytable)

Additionally, all the DDL objects are now regular `ClauseElement` objects just like any other SQLAlchemy expression object:

    from sqlalchemy.schema import CreateTable

    create = CreateTable(mytable)

    # dumps the CREATE TABLE as a string
    print(create)

    # executes the CREATE TABLE statement
    engine.execute(create)

and using the `sqlalchemy.ext.compiler` extension you can make your own:

    from sqlalchemy.schema import DDLElement
    from sqlalchemy.ext.compiler import compiles

    class AlterColumn(DDLElement):
        def __init__(self, column, cmd):
            self.column = column
            self.cmd = cmd

    @compiles(AlterColumn)
    def visit_alter_column(element, compiler, **kw):
        return "ALTER TABLE %s ALTER COLUMN %s %s ..." % (
            element.column.table.name,
            element.column.name,
            element.cmd,
        )

    engine.execute(AlterColumn(table.c.mycolumn, "SET DEFAULT 'test'"))

### Deprecated/Removed Schema Elements

The schema package has also been greatly streamlined. Many options and methods which were deprecated throughout 0.5 have been removed. Other little known accessors and methods have also been removed.

- the “owner” keyword argument is removed from `Table`. Use “schema” to represent any namespaces to be prepended to the table name.

- deprecated `MetaData.connect()` and `ThreadLocalMetaData.connect()` have been removed - send the “bind” attribute to bind a metadata.

- deprecated metadata.table_iterator() method removed (use sorted_tables)

- the “metadata” argument is removed from `DefaultGenerator` and subclasses, but remains locally present on `Sequence`, which is a standalone construct in DDL.

- deprecated `PassiveDefault` - use `DefaultClause`.

- Removed public mutability from `Index` and `Constraint` objects:

  - `ForeignKeyConstraint.append_element()`

  - `Index.append_column()`

  - `UniqueConstraint.append_column()`

  - `PrimaryKeyConstraint.add()`

  - `PrimaryKeyConstraint.remove()`

These should be constructed declaratively (i.e. in one construction).

- Other removed things:

  - `Table.key` (no idea what this was for)

  - `Column.bind` (get via column.table.bind)

  - `Column.metadata` (get via column.table.metadata)

  - `Column.sequence` (use column.default)

### Other Behavioral Changes

- `UniqueConstraint`, `Index`, `PrimaryKeyConstraint` all accept lists of column names or column objects as arguments.

- The `use_alter` flag on `ForeignKey` is now a shortcut option for operations that can be hand-constructed using the `DDL()` event system. A side effect of this refactor is that `ForeignKeyConstraint` objects with `use_alter=True` will not be emitted on SQLite, which does not support ALTER for foreign keys. This has no effect on SQLite’s behavior since SQLite does not actually honor FOREIGN KEY constraints.

- `Table.primary_key` is not assignable - use `table.append_constraint(PrimaryKeyConstraint(...))`

- A `Column` definition with a `ForeignKey` and no type, e.g. `Column(name, ForeignKey(sometable.c.somecol))` used to get the type of the referenced column. Now support for that automatic type inference is partial and may not work in all cases.

## Logging opened up

At the expense of a few extra method calls here and there, you can set log levels for INFO and DEBUG after an engine, pool, or mapper has been created, and logging will commence. The `isEnabledFor(INFO)` method is now called per-`Connection` and `isEnabledFor(DEBUG)` per-`ResultProxy` if already enabled on the parent connection. Pool logging sends to `log.info()` and `log.debug()` with no check - note that pool checkout/checkin is typically once per transaction.

## Reflection/Inspector API

The reflection system, which allows reflection of table columns via `Table('sometable', metadata, autoload=True)` has been opened up into its own fine-grained API, which allows direct inspection of database elements such as tables, columns, constraints, indexes, and more. This API expresses return values as simple lists of strings, dictionaries, and `TypeEngine` objects. The internals of `autoload=True` now build upon this system such that the translation of raw database information into `sqlalchemy.schema` constructs is centralized and the contract of individual dialects greatly simplified, vastly reducing bugs and inconsistencies across different backends.

To use an inspector:

    from sqlalchemy.engine.reflection import Inspector

    insp = Inspector.from_engine(my_engine)

    print(insp.get_schema_names())

the `from_engine()` method will in some cases provide a backend-specific inspector with additional capabilities, such as that of PostgreSQL which provides a `get_table_oid()` method:

    my_engine = create_engine("postgresql://...")
    pg_insp = Inspector.from_engine(my_engine)

    print(pg_insp.get_table_oid("my_table"))

## RETURNING Support

The `insert()`, `update()` and `delete()` constructs now support a `returning()` method, which corresponds to the SQL RETURNING clause as supported by PostgreSQL, Oracle, MS-SQL, and Firebird. It is not supported for any other backend at this time.

Given a list of column expressions in the same manner as that of a `select()` construct, the values of these columns will be returned as a regular result set:

    result = connection.execute(
        table.insert().values(data="some data").returning(table.c.id, table.c.timestamp)
    )
    row = result.first()
    print("ID:", row["id"], "Timestamp:", row["timestamp"])

The implementation of RETURNING across the four supported backends varies wildly, in the case of Oracle requiring an intricate usage of OUT parameters which are re-routed into a “mock” result set, and in the case of MS-SQL using an awkward SQL syntax. The usage of RETURNING is subject to limitations:

- it does not work for any “executemany()” style of execution. This is a limitation of all supported DBAPIs.

- Some backends, such as Oracle, only support RETURNING that returns a single row - this includes UPDATE and DELETE statements, meaning the update() or delete() construct must match only a single row, or an error is raised (by Oracle, not SQLAlchemy).

RETURNING is also used automatically by SQLAlchemy, when available and when not otherwise specified by an explicit `returning()` call, to fetch the value of newly generated primary key values for single-row INSERT statements. This means there’s no more “SELECT nextval(sequence)” pre- execution for insert statements where the primary key value is required. Truth be told, implicit RETURNING feature does incur more method overhead than the old “select nextval()” system, which used a quick and dirty cursor.execute() to get at the sequence value, and in the case of Oracle requires additional binding of out parameters. So if method/protocol overhead is proving to be more expensive than additional database round trips, the feature can be disabled by specifying `implicit_returning=False` to `create_engine()`.

## Type System Changes

### New Architecture

The type system has been completely reworked behind the scenes to provide two goals:

- Separate the handling of bind parameters and result row values, typically a DBAPI requirement, from the SQL specification of the type itself, which is a database requirement. This is consistent with the overall dialect refactor that separates database SQL behavior from DBAPI.

- Establish a clear and consistent contract for generating DDL from a `TypeEngine` object and for constructing `TypeEngine` objects based on column reflection.

Highlights of these changes include:

- The construction of types within dialects has been totally overhauled. Dialects now define publicly available types as UPPERCASE names exclusively, and internal implementation types using underscore identifiers (i.e. are private). The system by which types are expressed in SQL and DDL has been moved to the compiler system. This has the effect that there are much fewer type objects within most dialects. A detailed document on this architecture for dialect authors is in [source:/lib/sqlalc hemy/dialects/type_migration_guidelines.txt].

- Reflection of types now returns the exact UPPERCASE type within types.py, or the UPPERCASE type within the dialect itself if the type is not a standard SQL type. This means reflection now returns more accurate information about reflected types.

- User defined types that subclass `TypeEngine` and wish to provide `get_col_spec()` should now subclass `UserDefinedType`.

- The `result_processor()` method on all type classes now accepts an additional argument `coltype`. This is the DBAPI type object attached to cursor.description, and should be used when applicable to make better decisions on what kind of result-processing callable should be returned. Ideally result processor functions would never need to use `isinstance()`, which is an expensive call at this level.

### Native Unicode Mode

As more DBAPIs support returning Python unicode objects directly, the base dialect now performs a check upon the first connection which establishes whether or not the DBAPI returns a Python unicode object for a basic select of a VARCHAR value. If so, the `String` type and all subclasses (i.e. `Text`, `Unicode`, etc.) will skip the “unicode” check/conversion step when result rows are received. This offers a dramatic performance increase for large result sets. The “unicode mode” currently is known to work with:

- sqlite3 / pysqlite

- psycopg2 - SQLA 0.6 now uses the “UNICODE” type extension by default on each psycopg2 connection object

- pg8000

- cx_oracle (we use an output processor - nice feature !)

Other types may choose to disable unicode processing as needed, such as the `NVARCHAR` type when used with MS-SQL.

In particular, if porting an application based on a DBAPI that formerly returned non-unicode strings, the “native unicode” mode has a plainly different default behavior - columns that are declared as `String` or `VARCHAR` now return unicode by default whereas they would return strings before. This can break code which expects non-unicode strings. The psycopg2 “native unicode” mode can be disabled by passing `use_native_unicode=False` to `create_engine()`.

A more general solution for string columns that explicitly do not want a unicode object is to use a `TypeDecorator` that converts unicode back to utf-8, or whatever is desired:

    class UTF8Encoded(TypeDecorator):
        """Unicode type which coerces to utf-8."""

        impl = sa.VARCHAR

        def process_result_value(self, value, dialect):
            if isinstance(value, unicode):
                value = value.encode("utf-8")
            return value

Note that the `assert_unicode` flag is now deprecated. SQLAlchemy allows the DBAPI and backend database in use to handle Unicode parameters when available, and does not add operational overhead by checking the incoming type; modern systems like sqlite and PostgreSQL will raise an encoding error on their end if invalid data is passed. In those cases where SQLAlchemy does need to coerce a bind parameter from Python Unicode to an encoded string, or when the Unicode type is used explicitly, a warning is raised if the object is a bytestring. This warning can be suppressed or converted to an exception using the Python warnings filter documented at: https://docs.python.org/library/warnings.html

### Generic Enum Type

We now have an `Enum` in the `types` module. This is a string type that is given a collection of “labels” which constrain the possible values given to those labels. By default, this type generates a `VARCHAR` using the size of the largest label, and applies a CHECK constraint to the table within the CREATE TABLE statement. When using MySQL, the type by default uses MySQL’s ENUM type, and when using PostgreSQL the type will generate a user defined type using `CREATE TYPE <mytype> AS ENUM`. In order to create the type using PostgreSQL, the `name` parameter must be specified to the constructor. The type also accepts a `native_enum=False` option which will issue the VARCHAR/CHECK strategy for all databases. Note that PostgreSQL ENUM types currently don’t work with pg8000 or zxjdbc.

### Reflection Returns Dialect-Specific Types

Reflection now returns the most specific type possible from the database. That is, if you create a table using `String`, then reflect it back, the reflected column will likely be `VARCHAR`. For dialects that support a more specific form of the type, that’s what you’ll get. So a `Text` type would come back as `oracle.CLOB` on Oracle, a `LargeBinary` might be an `mysql.MEDIUMBLOB` etc. The obvious advantage here is that reflection preserves as much information possible from what the database had to say.

Some applications that deal heavily in table metadata may wish to compare types across reflected tables and/or non- reflected tables. There’s a semi-private accessor available on `TypeEngine` called `_type_affinity` and an associated comparison helper `_compare_type_affinity`. This accessor returns the “generic” `types` class which the type corresponds to:

    >>> String(50)._compare_type_affinity(postgresql.VARCHAR(50))
    True
    >>> Integer()._compare_type_affinity(mysql.REAL)
    False

### Miscellaneous API Changes

The usual “generic” types are still the general system in use, i.e. `String`, `Float`, `DateTime`. There’s a few changes there:

- Types no longer make any guesses as to default parameters. In particular, `Numeric`, `Float`, as well as subclasses NUMERIC, FLOAT, DECIMAL don’t generate any length or scale unless specified. This also continues to include the controversial `String` and `VARCHAR` types (although MySQL dialect will pre-emptively raise when asked to render VARCHAR with no length). No defaults are assumed, and if they are used in a CREATE TABLE statement, an error will be raised if the underlying database does not allow non-lengthed versions of these types.

- the `Binary` type has been renamed to `LargeBinary`, for BLOB/BYTEA/similar types. For `BINARY` and `VARBINARY`, those are present directly as `types.BINARY`, `types.VARBINARY`, as well as in the MySQL and MS-SQL dialects.

- `PickleType` now uses == for comparison of values when mutable=True, unless the “comparator” argument with a comparison function is specified to the type. If you are pickling a custom object you should implement an `__eq__()` method so that value-based comparisons are accurate.

- The default “precision” and “scale” arguments of Numeric and Float have been removed and now default to None. NUMERIC and FLOAT will be rendered with no numeric arguments by default unless these values are provided.

- DATE, TIME and DATETIME types on SQLite can now take optional “storage_format” and “regexp” argument. “storage_format” can be used to store those types using a custom string format. “regexp” allows to use a custom regular expression to match string values from the database.

- `__legacy_microseconds__` on SQLite `Time` and `DateTime` types is not supported anymore. You should use the new “storage_format” argument instead.

- `DateTime` types on SQLite now use by a default a stricter regular expression to match strings from the database. Use the new “regexp” argument if you are using data stored in a legacy format.

## ORM Changes

Upgrading an ORM application from 0.5 to 0.6 should require little to no changes, as the ORM’s behavior remains almost identical. There are some default argument and name changes, and some loading behaviors have been improved.

### New Unit of Work

The internals for the unit of work, primarily `topological.py` and `unitofwork.py`, have been completely rewritten and are vastly simplified. This should have no impact on usage, as all existing behavior during flush has been maintained exactly (or at least, as far as it is exercised by our testsuite and the handful of production environments which have tested it heavily). The performance of flush() now uses 20-30% fewer method calls and should also use less memory. The intent and flow of the source code should now be reasonably easy to follow, and the architecture of the flush is fairly open-ended at this point, creating room for potential new areas of sophistication. The flush process no longer has any reliance on recursion so flush plans of arbitrary size and complexity can be flushed. Additionally, the mapper’s “save” process, which issues INSERT and UPDATE statements, now caches the “compiled” form of the two statements so that callcounts are further dramatically reduced with very large flushes.

Any changes in behavior observed with flush versus earlier versions of 0.6 or 0.5 should be reported to us ASAP - we’ll make sure no functionality is lost.

### Changes to `query.update()` and `query.delete()`

- the ‘expire’ option on query.update() has been renamed to ‘fetch’, thus matching that of query.delete()

- `query.update()` and `query.delete()` both default to ‘evaluate’ for the synchronize strategy.

- the ‘synchronize’ strategy for update() and delete() raises an error on failure. There is no implicit fallback onto “fetch”. Failure of evaluation is based on the structure of criteria, so success/failure is deterministic based on code structure.

### `relation()` is officially named `relationship()`

This to solve the long running issue that “relation” means a “table or derived table” in relational algebra terms. The `relation()` name, which is less typing, will hang around for the foreseeable future so this change should be entirely painless.

### Subquery eager loading

A new kind of eager loading is added called “subquery” loading. This is a load that emits a second SQL query immediately after the first which loads full collections for all the parents in the first query, joining upwards to the parent using INNER JOIN. Subquery loading is used similarly to the current joined-eager loading, using the ``subqueryload()``` and ```subqueryload_all()``` options as well as the ```lazy='subquery'``` setting on ```relationship()``. The subquery load is usually much more efficient for loading many larger collections as it uses INNER JOIN unconditionally and also doesn’t re-load parent rows.

### ``eagerload()```, ```eagerload_all()``` is now ```joinedload()```, ```joinedload_all()``

To make room for the new subquery load feature, the existing ``eagerload()```/```eagerload_all()``` options are now superseded by ```joinedload()``` and ```joinedload_all()```. The old names will hang around for the foreseeable future just like ```relation()``.

### ``lazy=False|None|True|'dynamic'``` now accepts ```lazy='noload'|'joined'|'subquery'|'select'|'dynamic'``

Continuing on the theme of loader strategies opened up, the standard keywords for the ``lazy``` option on ```relationship()``` are now ```select``` for lazy loading (via a SELECT issued on attribute access), ```joined``` for joined-eager loading, ```subquery``` for subquery-eager loading, ```noload``` for no loading should occur, and ```dynamic``` for a “dynamic” relationship. The old ```True```, ```False```, ```None`` arguments are still accepted with the identical behavior as before.

### innerjoin=True on relation, joinedload

Joined-eagerly loaded scalars and collections can now be instructed to use INNER JOIN instead of OUTER JOIN. On PostgreSQL this is observed to provide a 300-600% speedup on some queries. Set this flag for any many-to-one which is on a NOT NULLable foreign key, and similarly for any collection where related items are guaranteed to exist.

At mapper level:

    mapper(Child, child)
    mapper(
        Parent,
        parent,
        properties={"child": relationship(Child, lazy="joined", innerjoin=True)},
    )

At query time level:

    session.query(Parent).options(joinedload(Parent.child, innerjoin=True)).all()

The `innerjoin=True` flag at the `relationship()` level will also take effect for any `joinedload()` option which does not override the value.

### Many-to-one Enhancements

- many-to-one relations now fire off a lazyload in fewer cases, including in most cases will not fetch the “old” value when a new one is replaced.

- many-to-one relation to a joined-table subclass now uses get() for a simple load (known as the “use_get” condition), i.e. `Related`->``Sub(Base)``, without the need to redefine the primaryjoin condition in terms of the base table. [ticket:1186]

- specifying a foreign key with a declarative column, i.e. `ForeignKey(MyRelatedClass.id)` doesn’t break the “use_get” condition from taking place [ticket:1492]

- relationship(), joinedload(), and joinedload_all() now feature an option called “innerjoin”. Specify `True` or `False` to control whether an eager join is constructed as an INNER or OUTER join. Default is `False` as always. The mapper options will override whichever setting is specified on relationship(). Should generally be set for many-to-one, not nullable foreign key relations to allow improved join performance. [ticket:1544]

- the behavior of joined eager loading such that the main query is wrapped in a subquery when LIMIT/OFFSET are present now makes an exception for the case when all eager loads are many-to-one joins. In those cases, the eager joins are against the parent table directly along with the limit/offset without the extra overhead of a subquery, since a many-to-one join does not add rows to the result.

  For example, in 0.5 this query:

      session.query(Address).options(eagerload(Address.user)).limit(10)

  would produce SQL like:

      SELECT * FROM
        (SELECT * FROM addresses LIMIT 10) AS anon_1
        LEFT OUTER JOIN users AS users_1 ON users_1.id = anon_1.addresses_user_id

  This because the presence of any eager loaders suggests that some or all of them may relate to multi-row collections, which would necessitate wrapping any kind of rowcount-sensitive modifiers like LIMIT inside of a subquery.

  In 0.6, that logic is more sensitive and can detect if all eager loaders represent many-to-ones, in which case the eager joins don’t affect the rowcount:

      SELECT * FROM addresses LEFT OUTER JOIN users AS users_1 ON users_1.id = addresses.user_id LIMIT 10

### Mutable Primary Keys with Joined Table Inheritance

A joined table inheritance config where the child table has a PK that foreign keys to the parent PK can now be updated on a CASCADE-capable database like PostgreSQL. `mapper()` now has an option `passive_updates=True` which indicates this foreign key is updated automatically. If on a non-cascading database like SQLite or MySQL/MyISAM, set this flag to `False`. A future feature enhancement will try to get this flag to be auto-configuring based on dialect/table style in use.

### Beaker Caching

A promising new example of Beaker integration is in `examples/beaker_caching`. This is a straightforward recipe which applies a Beaker cache within the result- generation engine of `Query`. Cache parameters are provided via `query.options()`, and allows full control over the contents of the cache. SQLAlchemy 0.6 includes improvements to the `Session.merge()` method to support this and similar recipes, as well as to provide significantly improved performance in most scenarios.

### Other Changes

- the “row tuple” object returned by `Query` when multiple column/entities are selected is now picklable as well as higher performing.

- `query.join()` has been reworked to provide more consistent behavior and more flexibility (includes [ticket:1537])

- `query.select_from()` accepts multiple clauses to produce multiple comma separated entries within the FROM clause. Useful when selecting from multiple-homed join() clauses.

- the “dont_load=True” flag on `Session.merge()` is deprecated and is now “load=False”.

- added “make_transient()” helper function which transforms a persistent/ detached instance into a transient one (i.e. deletes the instance_key and removes from any session.) [ticket:1052]

- the allow_null_pks flag on mapper() is deprecated and has been renamed to allow_partial_pks. It is turned “on” by default. This means that a row which has a non-null value for any of its primary key columns will be considered an identity. The need for this scenario typically only occurs when mapping to an outer join. When set to False, a PK that has NULLs in it will not be considered a primary key - in particular this means a result row will come back as None (or not be filled into a collection), and new in 0.6 also indicates that session.merge() won’t issue a round trip to the database for such a PK value. [ticket:1680]

- the mechanics of “backref” have been fully merged into the finer grained “back_populates” system, and take place entirely within the `_generate_backref()` method of `RelationProperty`. This makes the initialization procedure of `RelationProperty` simpler and allows easier propagation of settings (such as from subclasses of `RelationProperty`) into the reverse reference. The internal `BackRef()` is gone and `backref()` returns a plain tuple that is understood by `RelationProperty`.

- the keys attribute of `ResultProxy` is now a method, so references to it (`result.keys`) must be changed to method invocations (`result.keys()`)

- `ResultProxy.last_inserted_ids` is now deprecated, use `ResultProxy.inserted_primary_key` instead.

### Deprecated/Removed ORM Elements

Most elements that were deprecated throughout 0.5 and raised deprecation warnings have been removed (with a few exceptions). All elements that were marked “pending deprecation” are now deprecated and will raise a warning upon use.

- ‘transactional’ flag on sessionmaker() and others is removed. Use ‘autocommit=True’ to indicate ‘transactional=False’.

- ‘polymorphic_fetch’ argument on mapper() is removed. Loading can be controlled using the ‘with_polymorphic’ option.

- ‘select_table’ argument on mapper() is removed. Use ‘with_polymorphic=(“*”, <some selectable>)’ for this functionality.

- ‘proxy’ argument on synonym() is removed. This flag did nothing throughout 0.5, as the “proxy generation” behavior is now automatic.

- Passing a single list of elements to joinedload(), joinedload_all(), contains_eager(), lazyload(), defer(), and undefer() instead of multiple positional *args is deprecated.

- Passing a single list of elements to query.order_by(), query.group_by(), query.join(), or query.outerjoin() instead of multiple positional *args is deprecated.

- `query.iterate_instances()` is removed. Use `query.instances()`.

- `Query.query_from_parent()` is removed. Use the sqlalchemy.orm.with_parent() function to produce a “parent” clause, or alternatively `query.with_parent()`.

- `query._from_self()` is removed, use `query.from_self()` instead.

- the “comparator” argument to composite() is removed. Use “comparator_factory”.

- `RelationProperty._get_join()` is removed.

- the ‘echo_uow’ flag on Session is removed. Use logging on the “sqlalchemy.orm.unitofwork” name.

- `session.clear()` is removed. use `session.expunge_all()`.

- `session.save()`, `session.update()`, `session.save_or_update()` are removed. Use `session.add()` and `session.add_all()`.

- the “objects” flag on session.flush() remains deprecated.

- the “dont_load=True” flag on session.merge() is deprecated in favor of “load=False”.

- `ScopedSession.mapper` remains deprecated. See the usage recipe at https://www.sqlalchemy.org/trac/wiki/Usag eRecipes/SessionAwareMapper

- passing an `InstanceState` (internal SQLAlchemy state object) to `attributes.init_collection()` or `attributes.get_history()` is deprecated. These functions are public API and normally expect a regular mapped object instance.

- the ‘engine’ parameter to `declarative_base()` is removed. Use the ‘bind’ keyword argument.

## Extensions

### SQLSoup

SQLSoup has been modernized and updated to reflect common 0.5/0.6 capabilities, including well defined session integration. Please read the new docs at [https://www.sqlalchemy.org/docs/06/reference/ext/sqlsoup.html].

### Declarative

The `DeclarativeMeta` (default metaclass for `declarative_base`) previously allowed subclasses to modify `dict_` to add class attributes (e.g. columns). This no longer works, the `DeclarativeMeta` constructor now ignores `dict_`. Instead, the class attributes should be assigned directly, e.g. `cls.id=Column(...)`, or the MixIn class approach should be used instead of the metaclass approach.