Perfect, thanks! On Monday, 20 July 2015 10:49:26 UTC-4, Douglas Russell wrote: > > Apologies for cross-posting this, but I realised that this might be a > better place to get an answer than this Stackoverflow question > <http://stackoverflow.com/questions/31393824/polymorphic-self-referential-foreign-key-in-sqlalchemy> > > that I asked a week ago. > > 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_enginefrom > sqlalchemy.ext.declarative import declarative_basefrom 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 > <http://docs.sqlalchemy.org/en/latest/orm/relationship_persistence.html#rows-that-point-to-themselves-mutually-dependent-rows>, > > 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. > > Cheers, > > Douglas > >
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