=============================
What's New in SQLAlchemy 0.6?
=============================

.. admonition:: 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://ww
  w.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 :ticket:`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:

.. sourcecode:: text

    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:

  .. sourcecode:: sql

      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:

  .. sourcecode:: sql

      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.sqlalc
hemy.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 <https://www.sqlalchemy.org/docs/reference/ext/d
eclarative.html#mix-in-classes>`_ approach should be used
instead of the metaclass approach.

