Release: 1.2.8 | Release Date: May 28, 2018

# SQLAlchemy 1.2 Documentation

# Configuring Relationships

Relationships to other classes are done in the usual way, with the added feature that the class specified to `relationship()` 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 = relationship("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 = relationship(User, primaryjoin=user_id == User.id)

In addition to the main argument for `relationship()`, 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`:

    class User(Base):
        # ....
        addresses = relationship("Address",
                             order_by="desc(Address.email)",
                             primaryjoin="Address.user_id==User.id")

For the case where more than one module contains a class of the same name, string class names can also be specified as module-qualified paths within any of these string expressions:

    class User(Base):
        # ....
        addresses = relationship("myapp.model.address.Address",
                             order_by="desc(myapp.model.address.Address.email)",
                             primaryjoin="myapp.model.address.Address.user_id=="
                                            "myapp.model.user.User.id")

The qualified path can be any partial path that removes ambiguity between the names. For example, to disambiguate between `myapp.model.address.Address` and `myapp.model.lookup.Address`, we can specify `address.Address` or `lookup.Address`:

    class User(Base):
        # ....
        addresses = relationship("address.Address",
                             order_by="desc(address.Address.email)",
                             primaryjoin="address.Address.user_id=="
                                            "User.id")

New in version 0.8: module-qualified paths can be used when specifying string arguments with Declarative, in order to specify specific modules.

Two alternatives also exist to using string-based attributes. A lambda can also be used, which will be evaluated after all mappers have been configured:

    class User(Base):
        # ...
        addresses = relationship(lambda: Address,
                             order_by=lambda: desc(Address.email),
                             primaryjoin=lambda: Address.user_id==User.id)

Or, the relationship can be added to the class explicitly after the classes are available:

    User.addresses = relationship(Address,
                              primaryjoin=Address.user_id==User.id)

## Configuring Many-to-Many Relationships

Many-to-many relationships are also declared in the same way with declarative as with traditional mappings. The `secondary` argument to `relationship()` is as usual passed a `Table` object, which is typically declared in the traditional way. The `Table` usually shares the `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 = relationship("Keyword", secondary=keywords)

Like other `relationship()` arguments, a string is accepted as well, passing the string name of the table as defined in the `Base.metadata.tables` collection:

    class Author(Base):
        __tablename__ = 'authors'
        id = Column(Integer, primary_key=True)
        keywords = relationship("Keyword", secondary="keywords")

As with traditional mapping, its generally not a good idea to use a `Table` as the “secondary” argument which is also mapped to a class, unless the `relationship()` is declared with `viewonly=True`. Otherwise, the unit-of-work system may attempt duplicate INSERT and DELETE statements against the underlying table.