python - 将类与其自身相关联

Question

我有一个类Revision，Revision有如下定义:

class Revision(db.Model):
    id = db.Column(db.Unicode, primary_key=True)
    text = db.Column(db.Unicode)

此外，每个 Revision 可以有一个祖先，就像与另一个 Revision 的关系一样。每个 Revision 还应该跟踪任何后继者，即以它为祖先的 Revision。我尝试了以下两种方法:

ancestorId = db.Column(db.Unicode, db.ForeignKey("revision.id"))
successors = db.relationship("Revision", backref="ancestor", lazy="dynamic")

扔了:

sqlalchemy.exc.ArgumentError: Revision.successors and back-reference Revision.ancestor are both of the same direction <symbol 'ONETOMANY>.  Did you mean to set remote_side on the many-to-one side ?

和

successors = db.relationship("Revision", backref="ancestor", lazy="dynamic")

扔了:

sqlalchemy.exc.ArgumentError: Could not determine join condition between parent/child tables on relationship Revision.successors.  Specify a 'primaryjoin' expression.  If 'secondary' is present, 'secondaryjoin' is needed as well.

Answer 1

你要的叫adjacency list . SQLAlchemy 有一个 example在其示例目录中。重要的部分是 backref 的 remote_side 参数:

ancestorId = db.Column(db.Unicode, db.ForeignKey(id))
successors = db.relationship("Revision",
                             backref=db.backref("ancestor", remote_side=id),
                             lazy="dynamic")