SQLAlchemy relationship on many-to-many association table

Question

I am trying to build a relationship to another many-to-many relationship, the code looks like this:

from sqlalchemy import Column, Integer, ForeignKey, Table, ForeignKeyConstraint, create_engine
from sqlalchemy.orm import relationship, backref, scoped_session, sessionmaker
from sqlalchemy.ext.declarative import declarative_base

Base = declarative_base()

supervision_association_table = Table('supervision', Base.metadata,
    Column('supervisor_id', Integer, ForeignKey('supervisor.id'), primary_key=True),
    Column('client_id', Integer, ForeignKey('client.id'), primary_key=True)
)

class User(Base):
    __tablename__ = 'user'

    id = Column(Integer, primary_key=True)

class Supervisor(User):
    __tablename__ = 'supervisor'
    __mapper_args__ = {'polymorphic_identity': 'supervisor'}

    id = Column(Integer, ForeignKey('user.id'), primary_key = True)

    schedules = relationship("Schedule", backref='supervisor')

class Client(User):
    __tablename__ = 'client'
    __mapper_args__ = {'polymorphic_identity': 'client'}

    id = Column(Integer, ForeignKey('user.id'), primary_key = True)

    supervisor = relationship("Supervisor", secondary=supervision_association_table,
                                backref='clients')
    schedules = relationship("Schedule", backref="client")

class Schedule(Base):
    __tablename__ = 'schedule'
    __table_args__ = (
        ForeignKeyConstraint(['client_id', 'supervisor_id'], ['supervision.client_id', 'supervision.supervisor_id']),
    )

    id = Column(Integer, primary_key=True)
    client_id = Column(Integer, nullable=False)
    supervisor_id = Column(Integer, nullable=False)

engine = create_engine('sqlite:///temp.db')
db_session = scoped_session(sessionmaker(bind=engine))
Base.metadata.create_all(bind=engine)

What I want to do is to relate a schedule to a specific Client-Supervisor-relationship, though I have not found out how to do it. Going through the SQLAlchemy documentation I found a few hints, resulting in the ForeignKeyConstraint on the Schedule-Table.

How can I specify the relationship to have this association work?

what is the purpose of having separate supervision_association_table and then a Schedule class that has a one-to-one relationship to it ? Why not use one table, then use the association object pattern ? http://www.sqlalchemy.org/docs/orm/relationships.html#association-object. In any case you'd need to map supervision_associaiton_table here in order to create a chain of relationships to it and then to the related items if you keep with the two separate 'schedule' and 'supervision' tables. — zzzeek, Jan 22 '12 at 02:35
Can Client really have more than 1 supervisor? Your M-N table suggests *yes*, but the name of the reationship (supervisor) implies *No*. — van, Jan 22 '12 at 09:46
@zzzeek I want the schedule class to have a one-to-many relationship to the supervision, so that one pair of client-supervisor can have many different schedules. — dasmaze, Jan 22 '12 at 10:55
@van Your are right, the name is misleading. It is actually a many-to-many-relationship, so that each pair (as described in my comment above) can have several schedules. — dasmaze, Jan 22 '12 at 10:59

score 8 · Accepted Answer · edited Apr 07 '23 at 05:43

You need to map supervision_association_table so that you can create relationships to/from it.

I may be glossing over something here, but it seems like since you have many-to-many here you really can't have Client.schedules - if I say Client.schedules.append(some_schedule), which row in "supervision" is it pointing to?

The example below provides a read-only "rollup" accessor for those which joins the Schedule collections of each SupervisorAssociation. The association_proxy extension is used to conceal, when convenient, the details of the SupervisionAssociation object.

from sqlalchemy import Column, Integer, ForeignKey, Table, ForeignKeyConstraint, create_engine
from sqlalchemy.orm import relationship, backref, scoped_session, sessionmaker
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.ext.associationproxy import association_proxy
from itertools import chain

Base = declarative_base()

class SupervisionAssociation(Base):
    __tablename__ = 'supervision'

    supervisor_id = Column(Integer, ForeignKey('supervisor.id'), primary_key=True)
    client_id = Column(Integer, ForeignKey('client.id'), primary_key=True)

    supervisor = relationship("Supervisor", backref="client_associations")
    client = relationship("Client", backref="supervisor_associations")
    schedules = relationship("Schedule")

class User(Base):
    __tablename__ = 'user'

    id = Column(Integer, primary_key=True)

class Supervisor(User):
    __tablename__ = 'supervisor'
    __mapper_args__ = {'polymorphic_identity': 'supervisor'}

    id = Column(Integer, ForeignKey('user.id'), primary_key = True)

    clients = association_proxy("client_associations", "client", 
                        creator=lambda c: SupervisionAssociation(client=c))

    @property
    def schedules(self):
        return list(chain(*[c.schedules for c in self.client_associations]))

class Client(User):
    __tablename__ = 'client'
    __mapper_args__ = {'polymorphic_identity': 'client'}

    id = Column(Integer, ForeignKey('user.id'), primary_key = True)

    supervisors = association_proxy("supervisor_associations", "supervisor", 
                        creator=lambda s: SupervisionAssociation(supervisor=s))
    @property
    def schedules(self):
        return list(chain(*[s.schedules for s in self.supervisor_associations]))

class Schedule(Base):
    __tablename__ = 'schedule'
    __table_args__ = (
        ForeignKeyConstraint(['client_id', 'supervisor_id'], 
        ['supervision.client_id', 'supervision.supervisor_id']),
    )

    id = Column(Integer, primary_key=True)
    client_id = Column(Integer, nullable=False)
    supervisor_id = Column(Integer, nullable=False)
    client = association_proxy("supervisor_association", "client")

engine = create_engine('sqlite:///temp.db', echo=True)
db_session = scoped_session(sessionmaker(bind=engine))
Base.metadata.create_all(bind=engine)

c1, c2 = Client(), Client()
sp1, sp2 = Supervisor(), Supervisor()
sch1, sch2, sch3 = Schedule(), Schedule(), Schedule()

sp1.clients = [c1]
c2.supervisors = [sp2]
c2.supervisor_associations[0].schedules = [sch1, sch2]
c1.supervisor_associations[0].schedules = [sch3]

db_session.add_all([c1, c2, sp1, sp2, ])
db_session.commit()


print c1.schedules
print sp2.schedules

This is basically what I came to, having the schedules on the association table and mapping it with a class. I didn't use an association proxy, though I may do it. Thank you for this. — dasmaze, Jan 23 '12 at 21:15

SQLAlchemy relationship on many-to-many association table

1 Answers1

Linked