Polymorphic self referential foreign key in sqlalchemy

Question

I am trying to resolve a problem with a self-referential table with Joined Table Inheritance where there is a foreign key linking the inheritance relationships, but then also a case where a class has an additional reference to an instance of its parent. Best to go with a simplified example:

Class B inherits from Class A. Class B is linked to Class A by the id column through the Foreign Key in Class B. Class B also has a column ( a_id ) which references Class A which has nothing to do with the inheritance.

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

Base = declarative_base()


class A(Base):
    __tablename__ = 'a'
    satype = Column(String(50))
    __mapper_args__ = {
        'polymorphic_identity': 'a',
        'polymorphic_on': satype
    }

    id = Column(Integer, primary_key=True)


class B(A):
    __tablename__ = 'b'

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

    __mapper_args__ = {
        'polymorphic_identity': 'b'
    }

    a_id = Column(Integer, ForeignKey('a.id'))
    a = relationship('A', backref='b')

engine = create_engine('sqlite:///:memory:', echo=True)
Base.metadata.create_all(engine)
Session = sessionmaker(bind=engine)
session = Session()

As per the documentation , I would resolve the case where there are multiple ForeignKeys between tables by explicitly specifying in the relationship which was to be used.

class B(A):
    __tablename__ = 'b'

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

    __mapper_args__ = {
        'polymorphic_identity': 'b'
    }

    a_id = Column(Integer, ForeignKey('a.id'))
    # I know the primaryjoin is no longer needed in SA >= 0.8
    a = relationship('A', backref='b', foreign_keys=[a_id], primaryjoin=a_id==A.id)

I think the problem is that I don't seem to be able to figure out how to do the same for the polymorphic column id as I am not explicitly defining that relationship.

Answer 1

Thanks to Michael Bayer in the SA google groups for this answer:

The "mutually dependent foreign keys" document doesn't really apply to this case. What happens here is that B(A) requires a join from B to A, and then Ba requires a different one. Even though the conventions here make it clear which foreign key constraint is which, the mapper still needs them to be explicitly spelled out, that's like this:

class B(A):
    __tablename__ = 'b'

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

    __mapper_args__ = {
        'polymorphic_identity': 'b',
        'inherit_condition': id == A.id

    }

    a_id = Column(Integer, ForeignKey('a.id'))
    a = relationship(
        'A',
        backref='b', primaryjoin=A.id == a_id, remote_side=A.id)