I'm trying to set a relationship between User and UserEmail so that when I reference user.emails, the first email in the list is always the primary email. However, I'm getting `ProgrammingError: missing FROM-clause entry for table "app_user"` when I try to reference User.emails. How can I make this work? I might be going about this all wrong...

models.py

# coding=utf-8
from __future__ import unicode_literals, division

import sqlalchemy as sa
from sqlalchemy.orm import relationship, backref
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.ext.hybrid import hybrid_property
from werkzeug.security import generate_password_hash, check_password_hash

Base = declarative_base()


class User(Base):
    __tablename__ = "app_user"
    id = sa.Column(sa.Integer, primary_key=True)
    username = sa.Column(sa.String(255), unique=True, nullable=False)
    password_hash = sa.Column(sa.String(255))
    primary_email = sa.Column(sa.String(255))

    @hybrid_property
    def email(self):
        """
        This is the object associated with the primary email.
        """
        pes = [e for e in self.emails if self.primary_email == e.email]
        if len(pes) < 1:
            raise ValueError("missing primary email for user {}".format(self))
        if len(pes) > 1:
            raise ValueError("multiple primary emails for user {}".format(self))
        return pes[0]

    @email.expression
    def email(cls):
        return (
            UserEmail.query
            .join(cls)
            .where(cls.primary_email == UserEmail.email)
        )

    @property
    def password(self):
        raise AttributeError('password is not a readable attribute')

    @password.setter
    def password(self, password):
        self.password_hash = generate_password_hash(password)

    def verify_password(self, password):
        return check_password_hash(self.password_hash, password)

    def __unicode__(self):
        return self.username


class UserEmail(Base):
    __tablename__ = "app_user_email"
    id = sa.Column(sa.Integer, primary_key=True)
    email = sa.Column(sa.String(255), nullable=False, unique=True)
    user_id = sa.Column(sa.Integer, sa.ForeignKey(User.id))
    user = relationship(User, backref=backref(
        "emails",
        order_by=(User.primary_email == email),
    ))

    @property
    def primary(self):
        return self.user.primary_email == self.email

    def __unicode__(self):
        return self.email