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, backref
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. 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 the backref option:
class Parent(Base):
__tablename__ = 'parent'
id = Column(Integer, primary_key=True)
children = relationship("Child", backref="parent")
class Child(Base):
__tablename__ = 'child'
id = Column(Integer, primary_key=True)
parent_id = Column(Integer, ForeignKey('parent.id'))
Child will get a parent attribute with many-to-one semantics.
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 setting backref to the value "parents", which will place a one-to-many collection on the Child class:
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 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, backref="parent")
class Child(Base):
__tablename__ = 'child'
id = Column(Integer, primary_key=True)
parent_id = Column(Integer, ForeignKey('parent.id'))
Or to turn a one-to-many backref into one-to-one, use the backref() function to provide arguments for the reverse side:
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))
class Child(Base):
__tablename__ = 'child'
id = Column(Integer, primary_key=True)
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. The backref keyword 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")
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:
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.
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)
The bidirectional version adds backrefs to both relationships:
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_assocs")
class Parent(Base):
__tablename__ = 'left'
id = Column(Integer, primary_key=True)
children = relationship("Association", backref="parent")
class Child(Base):
__tablename__ = 'right'
id = Column(Integer, primary_key=True)
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.
Note
When using the association object pattern, it is advisable that the association-mapped table not be used as the secondary argument on a relationship() elsewhere, unless that relationship() contains the option viewonly set to True. SQLAlchemy otherwise may attempt to emit redundant INSERT and DELETE statements on the same table, if similar state is detected on the related attribute as well as the associated object.