Plugins  {@name=plugins}
======================

SQLAlchemy has a variety of extensions available which provide extra functionality to SA, either via explicit usage or by augmenting the core behavior.  Several of these extensions are designed to work together.

### declarative

**Author:** Mike Bayer<br/>
**Version:** 0.4.4 or greater

`declarative` intends to be a fully featured replacement for the very old `activemapper` extension.  Its goal is to redefine the organization of class, `Table`, and `mapper()` constructs such that they can all be defined "at once" underneath a class declaration.   Unlike `activemapper`, it does not redefine normal SQLAlchemy configurational semantics - regular `Column`, `relation()` and other schema or ORM constructs are used in almost all cases.

`declarative` is a so-called "micro declarative layer"; it does not generate table or column names and requires almost as fully verbose a configuration as that of straight tables and mappers.  As an alternative, the [Elixir](http://elixir.ematia.de/) project is a full community-supported declarative layer for SQLAlchemy, and is recommended for its active-record-like semantics, its convention-based configuration, and plugin capabilities.

SQLAlchemy object-relational configuration involves the usage of Table, mapper(), and class objects to define the three areas of configuration.
declarative moves these three types of configuration underneath the individual mapped class. Regular SQLAlchemy schema and ORM constructs are used
in most cases:

    {python}
    from sqlalchemy.ext.declarative import declarative_base
    
    Base = declarative_base()
    
    class SomeClass(Base):
        __tablename__ = 'some_table'
        id = Column('id', Integer, primary_key=True)
        name =  Column('name', String(50))

Above, the `declarative_base` callable produces a new base class from which all mapped classes inherit from. When the class definition is
completed, a new `Table` and `mapper()` have been generated, accessible via the `__table__` and `__mapper__` attributes on the
`SomeClass` class.

Attributes may be added to the class after its construction, and they will be added to the underlying `Table` and `mapper()` definitions as
appropriate:

    {python}
    SomeClass.data = Column('data', Unicode)
    SomeClass.related = relation(RelatedInfo)

Classes which are mapped explicitly using `mapper()` can interact freely with declarative classes. 

The `declarative_base` base class contains a `MetaData` object where newly defined `Table` objects are collected.  This is accessed via the ``metadata`` class level accessor, so to create tables we can say:

    {python}
    engine = create_engine('sqlite://')
    Base.metadata.create_all(engine)

The `Engine` created above may also be directly associated with the declarative base class using the `engine` keyword argument, where it will be associated with the underlying `MetaData` object and allow SQL operations involving that metadata and its tables to make use of that engine automatically:

    {python}
    Base = declarative_base(engine=create_engine('sqlite://'))

Or, as `MetaData` allows, at any time using the `bind` attribute:

    {python}
    Base.metadata.bind = create_engine('sqlite://')
 
The `declarative_base` can also receive a pre-created `MetaData` object, which allows a declarative setup to be associated with an already existing traditional collection of `Table` objects:

    {python}
    mymetadata = MetaData()
    Base = declarative_base(metadata=mymetadata)

Relations to other classes are done in the usual way, with the added feature that the class specified to `relation()` may be a string name. The
"class registry" associated with `Base` is used at mapper compilation time to resolve the name into the actual class object, which is expected to
have been defined once the mapper configuration is used:

    {python}
    class User(Base):
        __tablename__ = 'users'

        id = Column('id', Integer, primary_key=True)
        name = Column('name', String(50))
        addresses = relation("Address", backref="user")
    
    class Address(Base):
        __tablename__ = 'addresses'

        id = Column('id', Integer, primary_key=True)
        email = Column('email', String(50))
        user_id = Column('user_id', Integer, ForeignKey('users.id'))

Column constructs, since they are just that, are immediately usable, as below where we define a primary join condition on the `Address` class
using them:

    {python}
    class Address(Base)
        __tablename__ = 'addresses'

        id = Column('id', Integer, primary_key=True)
        email = Column('email', String(50))
        user_id = Column('user_id', Integer, ForeignKey('users.id'))
        user = relation(User, primaryjoin=user_id==User.id)

In addition to the main argument for `relation`, other arguments
which depend upon the columns present on an as-yet undefined class
may also be specified as strings.  These strings are evaluated as
Python expressions.  The full namespace available within this 
evaluation includes all classes mapped for this declarative base,
as well as the contents of the `sqlalchemy` package, including 
expression functions like `desc` and `func`:

    {python}
    class User(Base):
        # ....
        addresses = relation("Address", order_by="desc(Address.email)", 
            primaryjoin="Address.user_id==User.id")

As an alternative to string-based attributes, attributes may also be 
defined after all classes have been created.  Just add them to the target
class after the fact:

    {python}
    User.addresses = relation(Address, primaryjoin=Address.user_id==User.id)

Synonyms are one area where `declarative` needs to slightly change the usual SQLAlchemy configurational syntax. To define a
getter/setter which proxies to an underlying attribute, use `synonym` with the `instruments` argument:

    {python}
    class MyClass(Base):
        __tablename__ = 'sometable'
        
        _attr = Column('attr', String)
        
        def _get_attr(self):
            return self._some_attr
        def _set_attr(self, attr)
            self._some_attr = attr
        attr = synonym('_attr', instruments=property(_get_attr, _set_attr))
        
The above synonym is then usable as an instance attribute as well as a class-level expression construct:

    {python}
    x = MyClass()
    x.attr = "some value"
    session.query(MyClass).filter(MyClass.attr == 'some other value').all()

The `synonym_for` decorator can accomplish the same task:

    {python}
    class MyClass(Base):
        __tablename__ = 'sometable'
        
        _attr = Column('attr', String)

        @synonym_for('_attr')
        @property
        def attr(self):
            return self._some_attr

Similarly, `comparable_using` is a front end for the `comparable_property` ORM function:

    {python}
    class MyClass(Base):
        __tablename__ = 'sometable'

        name = Column('name', String)

        @comparable_using(MyUpperCaseComparator)
        @property
        def uc_name(self):
            return self.name.upper()

As an alternative to `__tablename__`, a direct `Table` construct may be used.  The `Column` objects, which in this case require their names, will be added to the mapping just like a regular mapping to a table:


    {python}
    class MyClass(Base):
        __table__ = Table('my_table', Base.metadata,
            Column('id', Integer, primary_key=True),
            Column('name', String(50))
        )

Other table-based attributes include `__table_args__`, which is
either a dictionary as in:

    {python}
    class MyClass(Base)
        __tablename__ = 'sometable'
        __table_args__ = {'mysql_engine':'InnoDB'}

or a dictionary-containing tuple in the form 
`(arg1, arg2, ..., {kwarg1:value, ...})`, as in:

    {python}
    class MyClass(Base)
        __tablename__ = 'sometable'
        __table_args__ = (ForeignKeyConstraint(['id'], ['remote_table.id']), {'autoload':True})

Mapper arguments are specified using the `__mapper_args__` class variable. Note that the column objects declared on the class are immediately
usable, as in this joined-table inheritance example:

    {python}
    class Person(Base):
        __tablename__ = 'people'
        id = Column('id', Integer, primary_key=True)
        discriminator = Column('type', String(50))
        __mapper_args__ = {'polymorphic_on':discriminator}
    
    class Engineer(Person):
        __tablename__ = 'engineers'
        __mapper_args__ = {'polymorphic_identity':'engineer'}
        id = Column('id', Integer, ForeignKey('people.id'), primary_key=True)
        primary_language = Column('primary_language', String(50))
        
For single-table inheritance, the `__tablename__` and `__table__` class variables are optional on a class when the class inherits from another
mapped class.

As a convenience feature, the `declarative_base()` sets a default constructor on classes which takes keyword arguments, and assigns them to the
named attributes:

    {python}
    e = Engineer(primary_language='python')

Note that `declarative` has no integration built in with sessions, and is only intended as an optional syntax for the regular usage of mappers
and Table objects. A typical application setup using `scoped_session` might look like:

    {python}
    engine = create_engine('postgres://scott:tiger@localhost/test')
    Session = scoped_session(sessionmaker(transactional=True, autoflush=False, bind=engine))
    Base = declarative_base()
    
Mapped instances then make usage of `Session` in the usual way.


### associationproxy

**Author:** Mike Bayer and Jason Kirtland<br/>
**Version:** 0.3.1 or greater

`associationproxy` is used to create a simplified, read/write view of a relationship.  It can be used to cherry-pick fields from a collection of related objects or to greatly simplify access to associated objects in an association relationship.

#### Simplifying Relations

Consider this "association object" mapping:

    {python}
    users_table = Table('users', metadata,
        Column('id', Integer, primary_key=True),
        Column('name', String(64)),
    )
    
    keywords_table = Table('keywords', metadata,
        Column('id', Integer, primary_key=True),
        Column('keyword', String(64))
    )

    userkeywords_table = Table('userkeywords', metadata,
        Column('user_id', Integer, ForeignKey("users.id"),
               primary_key=True),
        Column('keyword_id', Integer, ForeignKey("keywords.id"),
               primary_key=True)
    )

    class User(object):
        def __init__(self, name):
            self.name = name

    class Keyword(object):
        def __init__(self, keyword):
            self.keyword = keyword

    mapper(User, users_table, properties={
        'kw': relation(Keyword, secondary=userkeywords_table)
        })
    mapper(Keyword, keywords_table)

Above are three simple tables, modeling users, keywords and a many-to-many relationship between the two.  These ``Keyword`` objects are little more than a container for a name, and accessing them via the relation is awkward:

    {python}
    user = User('jek')
    user.kw.append(Keyword('cheese inspector'))
    print user.kw
    # [<__main__.Keyword object at 0xb791ea0c>]
    print user.kw[0].keyword
    # 'cheese inspector'
    print [keyword.keyword for keyword in u._keywords]
    # ['cheese inspector']

With ``association_proxy`` you have a "view" of the relation that contains just the `.keyword` of the related objects.  The proxy is a Python property, and unlike the mapper relation, is defined in your class:

    {python}
    from sqlalchemy.ext.associationproxy import association_proxy

    class User(object):
        def __init__(self, name):
            self.name = name

        # proxy the 'keyword' attribute from the 'kw' relation
        keywords = association_proxy('kw', 'keyword')

    # ...
    >>> user.kw
    [<__main__.Keyword object at 0xb791ea0c>]
    >>> user.keywords
    ['cheese inspector']
    >>> user.keywords.append('snack ninja')
    >>> user.keywords
    ['cheese inspector', 'snack ninja']
    >>> user.kw
    [<__main__.Keyword object at 0x9272a4c>, <__main__.Keyword object at 0xb7b396ec>]

The proxy is read/write.  New associated objects are created on demand when values are added to the proxy, and modifying or removing an entry through the proxy also affects the underlying collection.

- The association proxy property is backed by a mapper-defined relation, either a collection or scalar.
- You can access and modify both the proxy and the backing relation. Changes in one are immediate in the other.
- The proxy acts like the type of the underlying collection.  A list gets a list-like proxy, a dict a dict-like proxy, and so on.
- Multiple proxies for the same relation are fine.
- Proxies are lazy, and won't triger a load of the backing relation until they are accessed.
- The relation is inspected to determine the type of the related objects.
- To construct new instances, the type is called with the value being assigned, or key and value for dicts.
- A ``creator`` function can be used to create instances instead.

Above, the ``Keyword.__init__`` takes a single argument ``keyword``, which maps conveniently to the value being set through the proxy.  A ``creator`` function could have been used instead if more flexiblity was required.

Because the proxies are backed a regular relation collection, all of the usual hooks and patterns for using collections are still in effect.  The most convenient behavior is the automatic setting of "parent"-type relationships on assignment.  In the example above, nothing special had to be done to associate the Keyword to the User.  Simply adding it to the collection is sufficient.

#### Simplifying Association Object Relations

Association proxies are also useful for keeping [association objects](rel:datamapping_association) out the way during regular use.  For example, the  ``userkeywords`` table might have a bunch of auditing columns that need to get updated when changes are made- columns that are updated but seldom, if ever, accessed in your application.  A proxy can provide a very natural access pattern for the relation.

    {python}
    from sqlalchemy.ext.associationproxy import association_proxy

    # users_table and keywords_table tables as above, then:

    userkeywords_table = Table('userkeywords', metadata,
        Column('user_id', Integer, ForeignKey("users.id"), primary_key=True),
        Column('keyword_id', Integer, ForeignKey("keywords.id"), primary_key=True),
        # add some auditing columns
        Column('updated_at', DateTime, default=datetime.now),
        Column('updated_by', Integer, default=get_current_uid, onupdate=get_current_uid),
    )

    def _create_uk_by_keyword(keyword):
        """A creator function."""
        return UserKeyword(keyword=keyword)

    class User(object):
        def __init__(self, name):
            self.name = name
        keywords = association_proxy('user_keywords', 'keyword', creator=_create_uk_by_keyword)

    class Keyword(object):
        def __init__(self, keyword):
            self.keyword = keyword
        def __repr__(self):
            return 'Keyword(%s)' % repr(self.keyword)

    class UserKeyword(object):
        def __init__(self, user=None, keyword=None):
            self.user = user
            self.keyword = keyword

    mapper(User, users_table, properties={
        'user_keywords': relation(UserKeyword)
    })
    mapper(Keyword, keywords_table)
    mapper(UserKeyword, userkeywords_table, properties={
        'user': relation(User),
        'keyword': relation(Keyword),
    })


    user = User('log')
    kw1  = Keyword('new_from_blammo')

    # Adding a Keyword requires creating a UserKeyword association object
    user.user_keywords.append(UserKeyword(user, kw1))

    # And accessing Keywords requires traversing UserKeywords
    print user.user_keywords[0]
    # <__main__.UserKeyword object at 0xb79bbbec>

    print user.user_keywords[0].keyword
    # Keyword('new_from_blammo')

    # Lots of work.

    # It's much easier to go through the association proxy!
    for kw in (Keyword('its_big'), Keyword('its_heavy'), Keyword('its_wood')):
        user.keywords.append(kw)

    print user.keywords
    # [Keyword('new_from_blammo'), Keyword('its_big'), Keyword('its_heavy'), Keyword('its_wood')]


#### Building Complex Views

    {python}
    stocks = Table("stocks", meta,
       Column('symbol', String(10), primary_key=True),
       Column('description', String(100), nullable=False),
       Column('last_price', Numeric)
    )

    brokers = Table("brokers", meta,
       Column('id', Integer,primary_key=True),
       Column('name', String(100), nullable=False)
    )

    holdings = Table("holdings", meta,
      Column('broker_id', Integer, ForeignKey('brokers.id'), primary_key=True),
      Column('symbol', String(10), ForeignKey('stocks.symbol'), primary_key=True),
      Column('shares', Integer)
    )

Above are three tables, modeling stocks, their brokers and the number of shares of a stock held by each broker.  This situation is quite different from the association example above.  `shares` is a _property of the relation_, an important one that we need to use all the time.

For this example, it would be very convenient if `Broker` objects had a dictionary collection that mapped `Stock` instances to the shares held for each.  That's easy.

    {python}
    from sqlalchemy.ext.associationproxy import association_proxy
    from sqlalchemy.orm.collections import attribute_mapped_collection

    def _create_holding(stock, shares):
        """A creator function, constructs Holdings from Stock and share quantity."""
        return Holding(stock=stock, shares=shares)

    class Broker(object):
        def __init__(self, name):
            self.name = name

        holdings = association_proxy('by_stock', 'shares', creator=_create_holding)

    class Stock(object):
        def __init__(self, symbol, description=None):
            self.symbol = symbol
            self.description = description
            self.last_price = 0

    class Holding(object):
        def __init__(self, broker=None, stock=None, shares=0):
            self.broker = broker
            self.stock = stock
            self.shares = shares

    mapper(Stock, stocks_table)
    mapper(Broker, brokers_table, properties={
        'by_stock': relation(Holding,
            collection_class=attribute_mapped_collection('stock'))
    })
    mapper(Holding, holdings_table, properties={
        'stock': relation(Stock),
        'broker': relation(Broker)
    })

Above, we've set up the 'by_stock' relation collection to act as a dictionary, using the `.stock` property of each Holding as a key.

Populating and accessing that dictionary manually is slightly inconvenient because of the complexity of the Holdings association object:

    {python}
    stock = Stock('ZZK')
    broker = Broker('paj')

    broker.holdings[stock] = Holding(broker, stock, 10)
    print broker.holdings[stock].shares
    # 10

The `by_stock` proxy we've added to the `Broker` class hides the details of the `Holding` while also giving access to `.shares`:

    {python}
    for stock in (Stock('JEK'), Stock('STPZ')):
        broker.holdings[stock] = 123

    for stock, shares in broker.holdings.items():
        print stock, shares

    # lets take a peek at that holdings_table after committing changes to the db
    print list(holdings_table.select().execute())
    # [(1, 'ZZK', 10), (1, 'JEK', 123), (1, 'STEPZ', 123)]

Further examples can be found in the `examples/` directory in the SQLAlchemy distribution.

The `association_proxy` convenience function is not present in SQLAlchemy versions 0.3.1 through 0.3.7, instead instantiate the class directly:

    {python}
    from sqlalchemy.ext.associationproxy import AssociationProxy

    class Article(object):
       keywords = AssociationProxy('keyword_associations', 'keyword')


### orderinglist

**Author:** Jason Kirtland

`orderinglist` is a helper for mutable ordered relations.  It will intercept
list operations performed on a relation collection and automatically
synchronize changes in list position with an attribute on the related objects.
(See [advdatamapping_properties_entitycollections](rel:advdatamapping_properties_customcollections) for more information on the general pattern.)

Example: Two tables that store slides in a presentation.  Each slide
has a number of bullet points, displayed in order by the 'position'
column on the bullets table.  These bullets can be inserted and re-ordered
by your end users, and you need to update the 'position' column of all
affected rows when changes are made.

    {python}
    slides_table = Table('Slides', metadata,
                         Column('id', Integer, primary_key=True),
                         Column('name', String))

    bullets_table = Table('Bullets', metadata,
                          Column('id', Integer, primary_key=True),
                          Column('slide_id', Integer, ForeignKey('Slides.id')),
                          Column('position', Integer),
                          Column('text', String))

     class Slide(object):
         pass
     class Bullet(object):
         pass

     mapper(Slide, slides_table, properties={
           'bullets': relation(Bullet, order_by=[bullets_table.c.position])
     })
     mapper(Bullet, bullets_table)

The standard relation mapping will produce a list-like attribute on each Slide
containing all related Bullets, but coping with changes in ordering is totally
your responsibility.  If you insert a Bullet into that list, there is no
magic- it won't have a position attribute unless you assign it it one, and
you'll need to manually renumber all the subsequent Bullets in the list to
accommodate the insert.

An `orderinglist` can automate this and manage the 'position' attribute on all
related bullets for you.

    {python}        
    mapper(Slide, slides_table, properties={
           'bullets': relation(Bullet,
                               collection_class=ordering_list('position'),
                               order_by=[bullets_table.c.position])
    })
    mapper(Bullet, bullets_table)

    s = Slide()
    s.bullets.append(Bullet())
    s.bullets.append(Bullet())
    s.bullets[1].position
    >>> 1
    s.bullets.insert(1, Bullet())
    s.bullets[2].position
    >>> 2

Use the `ordering_list` function to set up the `collection_class` on relations
(as in the mapper example above).  This implementation depends on the list
starting in the proper order, so be SURE to put an order_by on your relation.

`ordering_list` takes the name of the related object's ordering attribute as
an argument.  By default, the zero-based integer index of the object's
position in the `ordering_list` is synchronized with the ordering attribute:
index 0 will get position 0, index 1 position 1, etc.  To start numbering at 1
or some other integer, provide `count_from=1`.

Ordering values are not limited to incrementing integers.  Almost any scheme
can implemented by supplying a custom `ordering_func` that maps a Python list
index to any value you require.  See the [module
documentation](rel:docstrings_sqlalchemy.ext.orderinglist) for more
information, and also check out the unit tests for examples of stepped
numbering, alphabetical and Fibonacci numbering.

### SqlSoup

**Author:** Jonathan Ellis

SqlSoup creates mapped classes on the fly from tables, which are automatically reflected from the database based on name.  It is essentially a nicer version of the "row data gateway" pattern.

    {python}
    >>> from sqlalchemy.ext.sqlsoup import SqlSoup
    >>> soup = SqlSoup('sqlite:///')

    >>> db.users.select(order_by=[db.users.c.name])
    [MappedUsers(name='Bhargan Basepair',email='basepair@example.edu',password='basepair',classname=None,admin=1),
     MappedUsers(name='Joe Student',email='student@example.edu',password='student',classname=None,admin=0)]

Full SqlSoup documentation is on the [SQLAlchemy Wiki](http://www.sqlalchemy.org/trac/wiki/SqlSoup).

