class ModelA(Base):
    __tablename__ = 'model_a'
    id = Column(String, primary_key=True)
    name = Column(String)

  class ModelB(Base):
    __tablename__ = 'model_b'
    id = Column(String, primary_key=True)
    number = Column(Integer)

  class ModelC(Base):
    __tablename__ = 'model_c'
    id = Column(String, primary_key=True)
    type = Column(String) # discriminator
    model = relationship(?)