Release: 1.2.12 current release | Release Date: September 19, 2018

# Basic Relationship Patterns¶

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()

## One To Many¶

A one to many relationship places a foreign key on the child table referencing the parent. 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 relationship() and connect the two using the 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 backref option may be used on a single relationship() instead of using back_populates:

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

## Many To One¶

Many to one places a foreign key in the parent table referencing the child. 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 relationship() and applying the 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 backref parameter may be applied to a single 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¶

One To One is essentially a bidirectional relationship with a scalar attribute on both sides. To achieve this, the 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 relationship.backref and backref() functions may be used in lieu of the relationship.back_populates approach; to specify uselist on a backref, use the 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¶

Many to Many adds an association table between two classes. The association table is indicated by the secondary argument to relationship(). Usually, the Table uses the MetaData object associated with the declarative base class, so that the 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 relationship.back_populates, and for each 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 backref parameter instead of relationship.back_populates, the backref will automatically use the same 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 secondary argument of 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")

### Deleting Rows from the Many to Many Table¶

A behavior which is unique to the secondary argument to relationship() is that the 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 Session.delete():

session.delete(somechild)

There are several possibilities here:

• If there is a 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 bidrectional; SQLAlchemy is strictly looking at every 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 passive_deletes directive on relationship(); see Using Passive Deletes for more details on this.

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

## Association Object¶

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 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 relationship.back_populates or 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 Association Proxy 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.

Warning

The association object pattern does not coordinate changes with a separate relationship that maps the association table as “secondary”.

Below, changes made to Parent.children will not be coordinated with changes made to Parent.child_associations or Child.parent_associations in Python; while all of these relationships will continue to function normally by themselves, changes on one will not show up in another until the Session is expired, which normally occurs automatically after Session.commit():

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)

Additionally, just as changes to one relationship aren’t reflected in the others automatically, writing the same data to both relationships will cause conflicting INSERT or DELETE statements as well, such as below where we establish the same relationship between a Parent and Child object twice:

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

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

It’s fine to use a mapping like the above if you know what you’re doing, though it may be a good idea to apply the viewonly=True parameter to the “secondary” relationship to avoid the issue of redundant changes being logged. However, to get a foolproof pattern that allows a simple two-object Parent->Child relationship while still using the association object pattern, use the association proxy extension as documented at Association Proxy.

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