flask-sqlalchemy one to one and many to one in same models

Question

I'm trying to make an invitation system where one User has an InvitationToken that can be sent to multiple invitees . Each User can have only one token (one-to-one), as well as be an invitee in multiple tokens (many-to-one).

This is what I have so far:

InvitationToken.py

class InvitationToken(db.Model, SerializeMixin, TimestampMixin):
    __tablename__ = "invitation_token"

    """
    The relationship to the User model. An InvitationToken can have many Users as invitees.
    """
    invitees = db.relationship(
        "User", back_populates="invitee_token", foreign_keys="User.invitee_token_id"
    )

    """
    The relationship to the User model. An InvitationToken can have only one User.
    """
    user_id = db.Column(UUID(as_uuid=True), db.ForeignKey("app_user.id"), index=True)
    user = db.relationship(
        "User",
        back_populates="invitation_token",
        foreign_keys=[user_id],
        uselist=False,
    )

User.py

class User(db.Model, TimestampMixin):
    __tablename__ = "app_user"

    """
    The relationship to the InvitationToken model. A User can have only one InvitationToken.
    """
    invitation_token_id = db.Column(
        UUID(as_uuid=True), db.ForeignKey("invitation_token.id"), index=True
    )
    invitation_token = db.relationship(
        "InvitationToken",
        back_populates="user",
        cascade="delete",
        foreign_keys=[invitation_token_id],
        uselist=False,
    )

    """
    The relationship to the InvitationToken model. A User can have only one InvitationToken as invitee.
    """
    invitee_token_id = db.Column(
        UUID(as_uuid=True), db.ForeignKey("invitation_token.id"), index=True
    )
    invitee_token = db.relationship(
        "InvitationToken",
        back_populates="invitees",
        foreign_keys=[invitee_token_id],
    )

I'm getting the following error with my current configuration:

sqlalchemy.exc.ArgumentError: InvitationToken.user and back-reference User.invitation_token are both of the same direction symbol('MANYTOONE').  Did you mean to set remote_side on the many-to-one side ?

What am I missing?

Answer 1

I found a solution, although I do not fully understand it. But it works:

class InvitationToken(db.Model, SerializeMixin, TimestampMixin):
    __tablename__ = "invitation_token"

    """
    The relationship to the User model. An InvitationToken can have many Users as invitees.
    """
    invitees = db.relationship(
        "User",
        back_populates="invitee_token",
        foreign_keys="User.invitee_token_id",  # Changed this
    )

    """
    The relationship to the User model. An InvitationToken can have only one User.
    """
    user_id = db.Column(UUID(as_uuid=True), db.ForeignKey("app_user.id"), index=True)
    user = db.relationship(
        "User",
        back_populates="invitation_token",
        foreign_keys="User.invitation_token_id",  # Changed this
        uselist=False,
    )

User was left unchanged. This got rid of the error and allowed me to run Flask and have the models behave how I wanted.