[sqlalchemy] One-to-many relationship between two subclasses with foreign key in parent classes

[02JanDal]

Hello,

I have two simple inheritance trees using joined table inheritance, see 
code below. In short, MembershipType inherits BaseShopItem and Membership 
inherits TransactionItem and there is a one-to-many relationship between 
BaseShopItem and TransactionItem.

This works well, but now I want to be able to get the membership type for a 
certain membership, as well as all memberships for a membership type. Since 
this information already is present (through TransactionItem.shop_item_id, 
thus between the parent classes) I do not want to introduce another foreign 
key between the subclasses. What is the most elegant way to represent this 
as a relationship?

I believe that composite secondary joins 
<https://docs.sqlalchemy.org/en/latest/orm/join_conditions.html#composite-secondary-joins>
 
are most likely what I need, but do not fully understand it and have 
therefore not been able to adapt it.

I'm using SQLAlchemy 1.3.1 with both SQLite and Postgres.

Thanks for your time,
Jan Dalheimer

Code: 

class BaseShopItem(db.Model):
    __tablename__ = 'shop_item'
    id = db.Column(db.Integer, primary_key=True)
    type = db.Column(db.String)

    transaction_item = db.relationship('TransactionItem', 
back_populates='transaction_items')
    
   __mapper_args__ = {
        'polymorphic_identity': 'shop_item',
        'polymorphic_on': type
    }

class MembershipType(ShopItem):
    __tablename__ = 'membership_type'
    id = db.Column(db.Integer, db.ForeignKey('shop_item.id'), 
primary_key=True)
    
   __mapper_args__ = {
        'polymorphic_identity': 'membership_type'
    }

class TransactionItem(db.Model):
    __tablename__ = 'transaction_item'
    id = db.Column(db.Integer, primary_key=True)
    type = db.Column(db.String)
    shop_item_id = db.Column(db.ForeignKey('shop_item.id'))

    shop_item = db.relationship('ShopItem', 
back_populates='transaction_items')
    
   __mapper_args__ = {
        'polymorphic_identity': 'transaction_item',
        'polymorphic_on': type
    }

class Membership(TransactionItem):
    __tablename__ = 'membership'
    id = db.Column(db.Integer, db.ForeignKey('shop_item.id'), 
primary_key=True)
    
   __mapper_args__ = {
        'polymorphic_identity': 'membership'
    }

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