class Post(db.Model):
    """An Post."""
id = db.Column(db.Integer, primary_key=True, nullable=False)
description = db.Column(db.Unicode(140), nullable=False)

@classmethod
def __declare_last__(cls):
    cls.entries = db.relationship(Entry, viewonly=True)

def attach_entries(self, entries):
    pass
Полиморфные отношения SQLAlchemy с конкретным наследованием