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Synopsis
========
SQLAlchemy object-relational configuration involves the usage of :class:`~sqlalchemy.schema.Table`,
:func:`~sqlalchemy.orm.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::
from sqlalchemy.ext.declarative import declarative_base
Base = declarative_base()
class SomeClass(Base):
__tablename__ = 'some_table'
id = Column(Integer, primary_key=True)
name = Column(String(50))
Above, the :func:`declarative_base` callable produces a new base class from which
all mapped classes inherit from. When the class definition is completed, a
new :class:`~sqlalchemy.schema.Table` and :class:`~sqlalchemy.orm.mapper` have been generated, accessible via the
``__table__`` and ``__mapper__`` attributes on the ``SomeClass`` class.
Defining Attributes
===================
:class:`~sqlalchemy.schema.Column` objects may be explicitly named,
including using a different name than the attribute in which they are associated.
The column will be assigned to the :class:`~sqlalchemy.schema.Table` using the
given name, and mapped to the class using the attribute name::
class SomeClass(Base):
__tablename__ = 'some_table'
id = Column("some_table_id", Integer, primary_key=True)
name = Column("name", String(50))
Otherwise, you may omit the names from the Column definitions.
Declarative will set the ``name`` attribute on the column when the class
is initialized::
class SomeClass(Base):
__tablename__ = 'some_table'
id = Column(Integer, primary_key=True)
name = Column(String(50))
Attributes may be added to the class after its construction, and they will be
added to the underlying :class:`~sqlalchemy.schema.Table` and :func:`~sqlalchemy.orm.mapper()` definitions as
appropriate::
SomeClass.data = Column('data', Unicode)
SomeClass.related = relation(RelatedInfo)
Classes which are mapped explicitly using :func:`~sqlalchemy.orm.mapper()` can interact freely
with declarative classes. It is recommended, though not required, that all tables
share the same underlying :class:`~sqlalchemy.schema.MetaData` object, so that
string-configured :class:`~sqlalchemy.schema.ForeignKey` references can be resolved without issue.
Association of Metadata and Engine
==================================
The :func:`declarative_base` base class contains a :class:`~sqlalchemy.schema.MetaData` object where newly
defined :class:`~sqlalchemy.schema.Table` objects are collected. This is accessed via the
:class:`~sqlalchemy.schema.MetaData` class level accessor, so to create tables we can say::
engine = create_engine('sqlite://')
Base.metadata.create_all(engine)
The :class:`~sqlalchemy.engine.base.Engine` created above may also be directly associated with the
declarative base class using the ``bind`` keyword argument, where it will be
associated with the underlying :class:`~sqlalchemy.schema.MetaData` object and allow SQL operations
involving that metadata and its tables to make use of that engine
automatically::
Base = declarative_base(bind=create_engine('sqlite://'))
Or, as :class:`~sqlalchemy.schema.MetaData` allows, at any time using the ``bind`` attribute::
Base.metadata.bind = create_engine('sqlite://')
The :func:`declarative_base` can also receive a pre-created :class:`~sqlalchemy.schema.MetaData` object,
which allows a declarative setup to be associated with an already existing
traditional collection of :class:`~sqlalchemy.schema.Table` objects::
mymetadata = MetaData()
Base = declarative_base(metadata=mymetadata)
Configuring Relations
=====================
Relations to other classes are done in the usual way, with the added feature
that the class specified to :func:`~sqlalchemy.orm.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::
class User(Base):
__tablename__ = 'users'
id = Column(Integer, primary_key=True)
name = Column(String(50))
addresses = relation("Address", backref="user")
class Address(Base):
__tablename__ = 'addresses'
id = Column(Integer, primary_key=True)
email = Column(String(50))
user_id = Column(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::
class Address(Base):
__tablename__ = 'addresses'
id = Column(Integer, primary_key=True)
email = Column(String(50))
user_id = Column(Integer, ForeignKey('users.id'))
user = relation(User, primaryjoin=user_id == User.id)
In addition to the main argument for :func:`~sqlalchemy.orm.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 :func:`~sqlalchemy.sql.expression.desc` and :attr:`~sqlalchemy.sql.expression.func`::
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::
User.addresses = relation(Address, primaryjoin=Address.user_id == User.id)
Configuring Many-to-Many Relations
==================================
There's nothing special about many-to-many with declarative. The ``secondary``
argument to :func:`~sqlalchemy.orm.relation` still requires a :class:`~sqlalchemy.schema.Table` object, not a declarative class.
The :class:`~sqlalchemy.schema.Table` should share the same :class:`~sqlalchemy.schema.MetaData` object used by the declarative base::
keywords = Table('keywords', Base.metadata,
Column('author_id', Integer, ForeignKey('authors.id')),
Column('keyword_id', Integer, ForeignKey('keywords.id'))
)
class Author(Base):
__tablename__ = 'authors'
id = Column(Integer, primary_key=True)
keywords = relation("Keyword", secondary=keywords)
You should generally **not** map a class and also specify its table in a many-to-many
relation, since the ORM may issue duplicate INSERT and DELETE statements.
Defining Synonyms
=================
Synonyms are introduced in :ref:`synonyms`. To define a getter/setter which
proxies to an underlying attribute, use :func:`~sqlalchemy.orm.synonym` with the
``descriptor`` argument::
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', descriptor=property(_get_attr, _set_attr))
The above synonym is then usable as an instance attribute as well as a
class-level expression construct::
x = MyClass()
x.attr = "some value"
session.query(MyClass).filter(MyClass.attr == 'some other value').all()
For simple getters, the :func:`synonym_for` decorator can be used in conjunction
with ``@property``::
class MyClass(Base):
__tablename__ = 'sometable'
_attr = Column('attr', String)
@synonym_for('_attr')
@property
def attr(self):
return self._some_attr
Similarly, :func:`comparable_using` is a front end for the :func:`~sqlalchemy.orm.comparable_property`
ORM function::
class MyClass(Base):
__tablename__ = 'sometable'
name = Column('name', String)
@comparable_using(MyUpperCaseComparator)
@property
def uc_name(self):
return self.name.upper()
Table Configuration
===================
As an alternative to ``__tablename__``, a direct :class:`~sqlalchemy.schema.Table` construct may be
used. The :class:`~sqlalchemy.schema.Column` objects, which in this case require their names, will be
added to the mapping just like a regular mapping to a table::
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::
class MyClass(Base):
__tablename__ = 'sometable'
__table_args__ = {'mysql_engine':'InnoDB'}
or a dictionary-containing tuple in the form
``(arg1, arg2, ..., {kwarg1:value, ...})``, as in::
class MyClass(Base):
__tablename__ = 'sometable'
__table_args__ = (ForeignKeyConstraint(['id'], ['remote_table.id']), {'autoload':True})
Mapper Configuration
====================
Mapper arguments are specified using the ``__mapper_args__`` class variable,
which is a dictionary that accepts the same names as the :class:`~sqlalchemy.orm.mapper`
function accepts as keywords::
class Widget(Base):
__tablename__ = 'widgets'
id = Column(Integer, primary_key=True)
__mapper_args__ = {'extension': MyWidgetExtension()}
Inheritance Configuration
=========================
Declarative supports all three forms of inheritance as intuitively
as possible. The ``inherits`` mapper keyword argument is not needed,
as declarative will determine this from the class itself. The various
"polymorphic" keyword arguments are specified using ``__mapper_args__``.
Joined Table Inheritance
~~~~~~~~~~~~~~~~~~~~~~~~
Joined table inheritance is defined as a subclass that defines its own
table::
class Person(Base):
__tablename__ = 'people'
id = Column(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(Integer, ForeignKey('people.id'), primary_key=True)
primary_language = Column(String(50))
Note that above, the ``Engineer.id`` attribute, since it shares the same
attribute name as the ``Person.id`` attribute, will in fact represent the ``people.id``
and ``engineers.id`` columns together, and will render inside a query as ``"people.id"``.
To provide the ``Engineer`` class with an attribute that represents only the
``engineers.id`` column, give it a different attribute name::
class Engineer(Person):
__tablename__ = 'engineers'
__mapper_args__ = {'polymorphic_identity': 'engineer'}
engineer_id = Column('id', Integer, ForeignKey('people.id'), primary_key=True)
primary_language = Column(String(50))
Single Table Inheritance
~~~~~~~~~~~~~~~~~~~~~~~~
Single table inheritance is defined as a subclass that does not have its
own table; you just leave out the ``__table__`` and ``__tablename__`` attributes::
class Person(Base):
__tablename__ = 'people'
id = Column(Integer, primary_key=True)
discriminator = Column('type', String(50))
__mapper_args__ = {'polymorphic_on': discriminator}
class Engineer(Person):
__mapper_args__ = {'polymorphic_identity': 'engineer'}
primary_language = Column(String(50))
When the above mappers are configured, the ``Person`` class is mapped to the ``people``
table *before* the ``primary_language`` column is defined, and this column will not be included
in its own mapping. When ``Engineer`` then defines the ``primary_language``
column, the column is added to the ``people`` table so that it is included in the mapping
for ``Engineer`` and is also part of the table's full set of columns.
Columns which are not mapped to ``Person`` are also excluded from any other
single or joined inheriting classes using the ``exclude_properties`` mapper argument.
Below, ``Manager`` will have all the attributes of ``Person`` and ``Manager`` but *not*
the ``primary_language`` attribute of ``Engineer``::
class Manager(Person):
__mapper_args__ = {'polymorphic_identity': 'manager'}
golf_swing = Column(String(50))
The attribute exclusion logic is provided by the ``exclude_properties`` mapper argument,
and declarative's default behavior can be disabled by passing an explicit
``exclude_properties`` collection (empty or otherwise) to the ``__mapper_args__``.
Concrete Table Inheritance
~~~~~~~~~~~~~~~~~~~~~~~~~~
Concrete is defined as a subclass which has its own table and sets the
``concrete`` keyword argument to ``True``::
class Person(Base):
__tablename__ = 'people'
id = Column(Integer, primary_key=True)
name = Column(String(50))
class Engineer(Person):
__tablename__ = 'engineers'
__mapper_args__ = {'concrete':True}
id = Column(Integer, primary_key=True)
primary_language = Column(String(50))
name = Column(String(50))
Usage of an abstract base class is a little less straightforward as it requires
usage of :func:`~sqlalchemy.orm.util.polymorphic_union`::
engineers = Table('engineers', Base.metadata,
Column('id', Integer, primary_key=True),
Column('name', String(50)),
Column('primary_language', String(50))
)
managers = Table('managers', Base.metadata,
Column('id', Integer, primary_key=True),
Column('name', String(50)),
Column('golf_swing', String(50))
)
punion = polymorphic_union({
'engineer':engineers,
'manager':managers
}, 'type', 'punion')
class Person(Base):
__table__ = punion
__mapper_args__ = {'polymorphic_on':punion.c.type}
class Engineer(Person):
__table__ = engineers
__mapper_args__ = {'polymorphic_identity':'engineer', 'concrete':True}
class Manager(Person):
__table__ = managers
__mapper_args__ = {'polymorphic_identity':'manager', 'concrete':True}
Class Usage
===========
As a convenience feature, the :func:`declarative_base` sets a default
constructor on classes which takes keyword arguments, and assigns them to the
named attributes::
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 :func:`~sqlalchemy.orm.scoped_session` might look
like::
engine = create_engine('postgres://scott:tiger@localhost/test')
Session = scoped_session(sessionmaker(autocommit=False,
autoflush=False,
bind=engine))
Base = declarative_base()
Mapped instances then make usage of :class:`~sqlalchemy.orm.session.Session` in the usual way.
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