A quick walkthrough of the basic relational patterns.

The imports used for each of the following sections is as follows:

from sqlalchemy import Table, Column, Integer, ForeignKey
from sqlalchemy.orm import relationship
from sqlalchemy.ext.declarative import declarative_base

Base = declarative_base()

A one to many relationship places a foreign key on the child table referencing the parent. :func:`_orm.relationship` is then specified on the parent, as referencing a collection of items represented by the child:

class Parent(Base):
    __tablename__ = 'parent'
    id = Column(Integer, primary_key=True)
    children = relationship("Child")

class Child(Base):
    __tablename__ = 'child'
    id = Column(Integer, primary_key=True)
    parent_id = Column(Integer, ForeignKey('parent.id'))

To establish a bidirectional relationship in one-to-many, where the "reverse" side is a many to one, specify an additional :func:`_orm.relationship` and connect the two using the :paramref:`_orm.relationship.back_populates` parameter:

class Parent(Base):
    __tablename__ = 'parent'
    id = Column(Integer, primary_key=True)
    children = relationship("Child", back_populates="parent")

class Child(Base):
    __tablename__ = 'child'
    id = Column(Integer, primary_key=True)
    parent_id = Column(Integer, ForeignKey('parent.id'))
    parent = relationship("Parent", back_populates="children")

Child will get a parent attribute with many-to-one semantics.

Alternatively, the :paramref:`_orm.relationship.backref` option may be used on a single :func:`_orm.relationship` instead of using :paramref:`_orm.relationship.back_populates`:

class Parent(Base):
    __tablename__ = 'parent'
    id = Column(Integer, primary_key=True)
    children = relationship("Child", backref="parent")

It is often the case that all Child objects should be deleted when their owning Parent is deleted. To configure this behavior, the delete cascade option described at :ref:`cascade_delete` is used. An additional option is that a Child object can itself be deleted when it is deassociated from its parent. This behavior is described at :ref:`cascade_delete_orphan`.

.. seealso::

    :ref:`cascade_delete`

    :ref:`passive_deletes`

    :ref:`cascade_delete_orphan`

Many to one places a foreign key in the parent table referencing the child. :func:`_orm.relationship` is declared on the parent, where a new scalar-holding attribute will be created:

class Parent(Base):
    __tablename__ = 'parent'
    id = Column(Integer, primary_key=True)
    child_id = Column(Integer, ForeignKey('child.id'))
    child = relationship("Child")

class Child(Base):
    __tablename__ = 'child'
    id = Column(Integer, primary_key=True)

Bidirectional behavior is achieved by adding a second :func:`_orm.relationship` and applying the :paramref:`_orm.relationship.back_populates` parameter in both directions:

class Parent(Base):
    __tablename__ = 'parent'
    id = Column(Integer, primary_key=True)
    child_id = Column(Integer, ForeignKey('child.id'))
    child = relationship("Child", back_populates="parents")

class Child(Base):
    __tablename__ = 'child'
    id = Column(Integer, primary_key=True)
    parents = relationship("Parent", back_populates="child")

Alternatively, the :paramref:`_orm.relationship.backref` parameter may be applied to a single :func:`_orm.relationship`, such as Parent.child:

class Parent(Base):
    __tablename__ = 'parent'
    id = Column(Integer, primary_key=True)
    child_id = Column(Integer, ForeignKey('child.id'))
    child = relationship("Child", backref="parents")

One To One is essentially a bidirectional relationship with a scalar attribute on both sides. To achieve this, the :paramref:`_orm.relationship.uselist` flag indicates the placement of a scalar attribute instead of a collection on the "many" side of the relationship. To convert one-to-many into one-to-one:

class Parent(Base):
    __tablename__ = 'parent'
    id = Column(Integer, primary_key=True)
    child = relationship("Child", uselist=False, back_populates="parent")

class Child(Base):
    __tablename__ = 'child'
    id = Column(Integer, primary_key=True)
    parent_id = Column(Integer, ForeignKey('parent.id'))
    parent = relationship("Parent", back_populates="child")

Or for many-to-one:

class Parent(Base):
    __tablename__ = 'parent'
    id = Column(Integer, primary_key=True)
    child_id = Column(Integer, ForeignKey('child.id'))
    child = relationship("Child", back_populates="parent")

class Child(Base):
    __tablename__ = 'child'
    id = Column(Integer, primary_key=True)
    parent = relationship("Parent", back_populates="child", uselist=False)

As always, the :paramref:`_orm.relationship.backref` and :func:`.backref` functions may be used in lieu of the :paramref:`_orm.relationship.back_populates` approach; to specify uselist on a backref, use the :func:`.backref` function:

from sqlalchemy.orm import backref

class Parent(Base):
    __tablename__ = 'parent'
    id = Column(Integer, primary_key=True)
    child_id = Column(Integer, ForeignKey('child.id'))
    child = relationship("Child", backref=backref("parent", uselist=False))

Many to Many adds an association table between two classes. The association table is indicated by the :paramref:`_orm.relationship.secondary` argument to :func:`_orm.relationship`. Usually, the :class:`_schema.Table` uses the :class:`_schema.MetaData` object associated with the declarative base class, so that the :class:`_schema.ForeignKey` directives can locate the remote tables with which to link:

association_table = Table('association', Base.metadata,
    Column('left_id', Integer, ForeignKey('left.id')),
    Column('right_id', Integer, ForeignKey('right.id'))
)

class Parent(Base):
    __tablename__ = 'left'
    id = Column(Integer, primary_key=True)
    children = relationship("Child",
                    secondary=association_table)

class Child(Base):
    __tablename__ = 'right'
    id = Column(Integer, primary_key=True)

For a bidirectional relationship, both sides of the relationship contain a collection. Specify using :paramref:`_orm.relationship.back_populates`, and for each :func:`_orm.relationship` specify the common association table:

association_table = Table('association', Base.metadata,
    Column('left_id', Integer, ForeignKey('left.id')),
    Column('right_id', Integer, ForeignKey('right.id'))
)

class Parent(Base):
    __tablename__ = 'left'
    id = Column(Integer, primary_key=True)
    children = relationship(
        "Child",
        secondary=association_table,
        back_populates="parents")

class Child(Base):
    __tablename__ = 'right'
    id = Column(Integer, primary_key=True)
    parents = relationship(
        "Parent",
        secondary=association_table,
        back_populates="children")

When using the :paramref:`_orm.relationship.backref` parameter instead of :paramref:`_orm.relationship.back_populates`, the backref will automatically use the same :paramref:`_orm.relationship.secondary` argument for the reverse relationship:

association_table = Table('association', Base.metadata,
    Column('left_id', Integer, ForeignKey('left.id')),
    Column('right_id', Integer, ForeignKey('right.id'))
)

class Parent(Base):
    __tablename__ = 'left'
    id = Column(Integer, primary_key=True)
    children = relationship("Child",
                    secondary=association_table,
                    backref="parents")

class Child(Base):
    __tablename__ = 'right'
    id = Column(Integer, primary_key=True)

The :paramref:`_orm.relationship.secondary` argument of :func:`_orm.relationship` also accepts a callable that returns the ultimate argument, which is evaluated only when mappers are first used. Using this, we can define the association_table at a later point, as long as it's available to the callable after all module initialization is complete:

class Parent(Base):
    __tablename__ = 'left'
    id = Column(Integer, primary_key=True)
    children = relationship("Child",
                    secondary=lambda: association_table,
                    backref="parents")

With the declarative extension in use, the traditional "string name of the table" is accepted as well, matching the name of the table as stored in Base.metadata.tables:

class Parent(Base):
    __tablename__ = 'left'
    id = Column(Integer, primary_key=True)
    children = relationship("Child",
                    secondary="association",
                    backref="parents")

A behavior which is unique to the :paramref:`_orm.relationship.secondary` argument to :func:`_orm.relationship` is that the :class:`_schema.Table` which is specified here is automatically subject to INSERT and DELETE statements, as objects are added or removed from the collection. There is no need to delete from this table manually. The act of removing a record from the collection will have the effect of the row being deleted on flush:

# row will be deleted from the "secondary" table
# automatically
myparent.children.remove(somechild)

A question which often arises is how the row in the "secondary" table can be deleted when the child object is handed directly to :meth:`.Session.delete`:

session.delete(somechild)

There are several possibilities here:

• If there is a :func:`_orm.relationship` from Parent to Child, but there is not a reverse-relationship that links a particular Child to each Parent, SQLAlchemy will not have any awareness that when deleting this particular Child object, it needs to maintain the "secondary" table that links it to the Parent. No delete of the "secondary" table will occur.
• If there is a relationship that links a particular Child to each Parent, suppose it's called Child.parents, SQLAlchemy by default will load in the Child.parents collection to locate all Parent objects, and remove each row from the "secondary" table which establishes this link. Note that this relationship does not need to be bidirectional; SQLAlchemy is strictly looking at every :func:`_orm.relationship` associated with the Child object being deleted.
• A higher performing option here is to use ON DELETE CASCADE directives with the foreign keys used by the database. Assuming the database supports this feature, the database itself can be made to automatically delete rows in the "secondary" table as referencing rows in "child" are deleted. SQLAlchemy can be instructed to forego actively loading in the Child.parents collection in this case using the :paramref:`_orm.relationship.passive_deletes` directive on :func:`_orm.relationship`; see :ref:`passive_deletes` for more details on this.

Note again, these behaviors are only relevant to the :paramref:`_orm.relationship.secondary` option used with :func:`_orm.relationship`. If dealing with association tables that are mapped explicitly and are not present in the :paramref:`_orm.relationship.secondary` option of a relevant :func:`_orm.relationship`, cascade rules can be used instead to automatically delete entities in reaction to a related entity being deleted - see :ref:`unitofwork_cascades` for information on this feature.

.. seealso::

    :ref:`cascade_delete_many_to_many`

    :ref:`passive_deletes_many_to_many`

The association object pattern is a variant on many-to-many: it's used when your association table contains additional columns beyond those which are foreign keys to the left and right tables. Instead of using the :paramref:`_orm.relationship.secondary` argument, you map a new class directly to the association table. The left side of the relationship references the association object via one-to-many, and the association class references the right side via many-to-one. Below we illustrate an association table mapped to the Association class which includes a column called extra_data, which is a string value that is stored along with each association between Parent and Child:

class Association(Base):
    __tablename__ = 'association'
    left_id = Column(Integer, ForeignKey('left.id'), primary_key=True)
    right_id = Column(Integer, ForeignKey('right.id'), primary_key=True)
    extra_data = Column(String(50))
    child = relationship("Child")

class Parent(Base):
    __tablename__ = 'left'
    id = Column(Integer, primary_key=True)
    children = relationship("Association")

class Child(Base):
    __tablename__ = 'right'
    id = Column(Integer, primary_key=True)

As always, the bidirectional version makes use of :paramref:`_orm.relationship.back_populates` or :paramref:`_orm.relationship.backref`:

class Association(Base):
    __tablename__ = 'association'
    left_id = Column(Integer, ForeignKey('left.id'), primary_key=True)
    right_id = Column(Integer, ForeignKey('right.id'), primary_key=True)
    extra_data = Column(String(50))
    child = relationship("Child", back_populates="parents")
    parent = relationship("Parent", back_populates="children")

class Parent(Base):
    __tablename__ = 'left'
    id = Column(Integer, primary_key=True)
    children = relationship("Association", back_populates="parent")

class Child(Base):
    __tablename__ = 'right'
    id = Column(Integer, primary_key=True)
    parents = relationship("Association", back_populates="child")

Working with the association pattern in its direct form requires that child objects are associated with an association instance before being appended to the parent; similarly, access from parent to child goes through the association object:

# create parent, append a child via association
p = Parent()
a = Association(extra_data="some data")
a.child = Child()
p.children.append(a)

# iterate through child objects via association, including association
# attributes
for assoc in p.children:
    print(assoc.extra_data)
    print(assoc.child)

To enhance the association object pattern such that direct access to the Association object is optional, SQLAlchemy provides the :ref:`associationproxy_toplevel` extension. This extension allows the configuration of attributes which will access two "hops" with a single access, one "hop" to the associated object, and a second to a target attribute.

class Association(Base):
    __tablename__ = 'association'

    left_id = Column(Integer, ForeignKey('left.id'), primary_key=True)
    right_id = Column(Integer, ForeignKey('right.id'), primary_key=True)
    extra_data = Column(String(50))

    child = relationship("Child", backref="parent_associations")
    parent = relationship("Parent", backref="child_associations")

class Parent(Base):
    __tablename__ = 'left'
    id = Column(Integer, primary_key=True)

    children = relationship("Child", secondary="association")

class Child(Base):
    __tablename__ = 'right'
    id = Column(Integer, primary_key=True)

p1 = Parent()
c1 = Child()
p1.children.append(c1)

# redundant, will cause a duplicate INSERT on Association
p1.child_associations.append(Association(child=c1))

Many of the examples in the preceding sections illustrate mappings where the various :func:`_orm.relationship` constructs refer to their target classes using a string name, rather than the class itself:

class Parent(Base):
    # ...

    children = relationship("Child", back_populates="parent")

class Child(Base):
    # ...

    parent = relationship("Parent", back_populates="children")

These string names are resolved into classes in the mapper resolution stage, which is an internal process that occurs typically after all mappings have been defined and is normally triggered by the first usage of the mappings themselves. The :class:`_orm.registry` object is the container in which these names are stored and resolved to the mapped classes they refer towards.

In addition to the main class argument for :func:`_orm.relationship`, other arguments which depend upon the columns present on an as-yet undefined class may also be specified either as Python functions, or more commonly as strings. For most of these arguments except that of the main argument, string inputs are evaluated as Python expressions using Python's built-in eval() function, as they are intended to recieve complete SQL 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:`_sql.desc` and :attr:`_functions.func`:

class Parent(Base):
    # ...

    children = relationship(
        "Child",
        order_by="desc(Child.email_address)",
        primaryjoin="Parent.id == Child.parent_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 Parent(Base):
    # ...

    children = relationship(
        "myapp.mymodel.Child",
        order_by="desc(myapp.mymodel.Child.email_address)",
        primaryjoin="myapp.mymodel.Parent.id == myapp.mymodel.Child.parent_id"
    )

The qualified path can be any partial path that removes ambiguity between the names. For example, to disambiguate between myapp.model1.Child and myapp.model2.Child, we can specify model1.Child or model2.Child:

class Parent(Base):
    # ...

    children = relationship(
        "model1.Child",
        order_by="desc(mymodel1.Child.email_address)",
        primaryjoin="Parent.id == model1.Child.parent_id"
    )

The :func:`_orm.relationship` construct also accepts Python functions or lambdas as input for these arguments. This has the advantage of providing more compile-time safety and better support for IDEs and PEP 484 scenarios.

A Python functional approach might look like the following:

from sqlalchemy import desc

def _resolve_child_model():
     from myapplication import Child
     return Child

class Parent(Base):
    # ...

    children = relationship(
        _resolve_child_model(),
        order_by=lambda: desc(_resolve_child_model().email_address),
        primaryjoin=lambda: Parent.id == _resolve_child_model().parent_id
    )

The full list of parameters which accept Python functions/lambdas or strings that will be passed to eval() are:

• :paramref:`_orm.relationship.order_by`
• :paramref:`_orm.relationship.primaryjoin`
• :paramref:`_orm.relationship.secondaryjoin`
• :paramref:`_orm.relationship.secondary`
• :paramref:`_orm.relationship.remote_side`
• :paramref:`_orm.relationship.foreign_keys`
• :paramref:`_orm.relationship._user_defined_foreign_keys`

.. versionchanged:: 1.3.16

    Prior to SQLAlchemy 1.3.16, the main :paramref:`_orm.relationship.argument`
    to :func:`_orm.relationship` was also evaluated throught ``eval()``   As of
    1.3.16 the string name is resolved from the class resolver directly without
    supporting custom Python expressions.

It should also be noted that in a similar way as described at :ref:`orm_declarative_table_adding_columns`, any :class:`_orm.MapperProperty` construct can be added to a declarative base mapping at any time. If we wanted to implement this :func:`_orm.relationship` after the Address class were available, we could also apply it afterwards:

# first, module A, where Child has not been created yet,
# we create a Parent class which knows nothing about Child

class Parent(Base):
    # ...


#... later, in Module B, which is imported after module A:

class Child(Base):
    # ...

from module_a import Parent

# assign the User.addresses relationship as a class variable.  The
# declarative base class will intercept this and map the relationship.
Parent.children = relationship(
    Child,
    primaryjoin=Child.parent_id==Parent.id
)

Many-to-many relationships include a reference to an additional, typically non-mapped :class:`_schema.Table` object that is typically present in the :class:`_schema.MetaData` collection referred towards by the :class:`_orm.registry`. The late-evaluation system also includes support for having this attribute be specified as a string argument which will be resolved from this :class:`_schema.MetaData` collection. Below we specify an association table keyword_author, sharing the :class:`_schema.MetaData` collection associated with our declarative base and its :class:`_orm.registry`. We can then refer to this :class:`_schema.Table` by name in the :paramref:`_orm.relationship.secondary` parameter:

keyword_author = Table(
    'keyword_author', 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="keyword_author")

For additional detail on many-to-many relationships see the section :ref:`relationships_many_to_many`.