polymorphic association sqlalchemy

generic.py

from sqlalchemy import event, and_
from sqlalchemy.orm import relationship, foreign, remote, backref


class PostFavorites(db.Model):
    id = db.Column(db.Integer, primary_key=True, nullable=False)

    user_id = db.Column(db.Integer, nullable=False)

    post_id = db.Column(db.Integer, nullable=False)

    post_type = db.Column(db.Integer, nullable=False)

    @property
    def post(self):
        return getattr(self, "parent_%s" % self.post_type)


class HasPostFavorite(object):
    """HasPostFavorite mixin, creates relationship to post association"""


@event.listens_for(HasPostFavorite, "mapper_configured", propagate=True)
def setup_listener(mapper, class_):
    discriminator = class_.type
    class_.post_favorite = relationship(
        PostFavorites,
        primaryjoin=and_(
            class_.id == foreign(remote(PostFavorites.post_id)),
            PostFavorites.post_type == discriminator
        ),
        backref=backref(
            "parent_%s" % discriminator,
            primaryjoin=remote(class_.id) == foreign(PostFavorites.post_id)
        )
    )

    @event.listens_for(class_.post_favorite, "append")
    def append_address(target, value, initiator):
        value.post_type = discriminator