На PHP не знам но на пътхон бих направил нещо от сорта:
class Debit(Base):
__tablename__ = 'debit'
class Kredit(Base):
__tablename__ = 'kredit'
class Order(Base):
__tablename__ = 'order'
id = Column(Integer, primary_key=True)
paid = Column(Boolean, nullable=False, default=False)
debit_id = Column(Integer, ForeignKey('debit.id'), nullable=True, unique=True)
debit = relationship( 'Debit', backref="my_debit")
credit_id = Column(Integer, ForeignKey('credit.id'), nullable=True, unique=True)
credit = relationship( 'Kredit', backref="my_credit")