使用SQLAlchemy具有多个自引用外键

Question

What if I had something like a double linked list in a relational database, for example:

node_id    left_id    right_id

1          null       2
2          1          3
3          2          null

Then I have some SQLAlchemy code like the following:

class NodeClass(Base):
    __tablename__ = 'nodes_table'
    node_id = Column(Integer, primary_key=True)
    left_id = Column(Integer, ForeignKey('nodes_table.node_id'))
    right_id = Column(Integer, ForeignKey('nodes_table.node_id'))
    left = relationship('NodeClass') # Wrong
    right = relationship('NodeClass') # Wrong

If I have node_id 2, and I call NodeClass.left I would like to receive node_id 1 in return. How can I configure the SQLAlchemy relationships to behave this way?

UPDATE:

I will give a second example. Consider a table of people, and each person has a mother and a father.

person_id    mother_id    father_id

1            null         null
2            null         null
3            1            2

The SQLAlchemy code:

class PersonClass(Base):
    __tablename__ = 'persons_table'
    person_id = Column(Integer, primary_key=True)
    mother_id = Column(Integer, ForeignKey('persons_table.person_id'))
    father_id = Column(Integer, ForeignKey('persons_table.person_id'))
    mother = relation('PersonClass') # Wrong
    father = relation('PersonClass') # Wrong

Answer 1

The code below shows how to configure the relationship:

class NodeClass(Base):
    __tablename__ = 'nodes_table'
    node_id = Column(Integer, primary_key=True)

    left_id = Column(Integer, ForeignKey('nodes_table.node_id'))
    left = relationship('NodeClass', remote_side=[node_id],
            primaryjoin=('NodeClass.left_id==NodeClass.node_id'),
            backref=backref("right", uselist=False),
            #lazy="joined", join_depth=9,
            )

But few things should be noted:

• only one end of the relationship is stored, the other is inferred. This might not be what you want, but it is much more simple to manage, and it is enough to set one side only myNode.left = myOtherNode and the other ( right ) will be set automatically (because of configured backref )
• if both ends are stored ( right and left ), then
  • both ends would need to be set in code and one has to ensure they are consistent, which might not be a trivial task
  • the insert of two nodes that are linked would require insert-1, insert-2, update-1 in case your primary key is computed on the database, as it is unknown during the first insert .

UPDATE: Sample code to the UPDATE part of the question (but still using the original class name). One needs only to specify the primaryjoin and uselist=False :

class NodeClass(Base):
    __tablename__ = 'nodes_table'
    node_id = Column(Integer, primary_key=True)

    left_id = Column(Integer, ForeignKey('nodes_table.node_id'))
    right_id = Column(Integer, ForeignKey('nodes_table.node_id'))

    left = relationship('NodeClass', primaryjoin = ('NodeClass.left_id == NodeClass.node_id'), use_list=False)
    right = relationship('NodeClass', primaryjoin = ('NodeClass.right_id == NodeClass.node_id'), use_list=False)