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 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'

    }

[Mike Bayer]

On Sat, Mar 23, 2019 at 12:39 PM 02JanDal <j...@dalheimer.de> wrote:
>
> 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?

Are you looking just to have Membership.transaction_items with the
TransactionItem objects filtered out and vice versa ? I think you
can use relationship() directly, Membership.membership_types =
relationship("MembershipType") / MembershipType.membership =
relationship("Membership") . You just need to be careful not to
modify both levels of relationship at once so you'd probably make
these viewonly. Alternatively, just build accessor methods with
@property that iterate Membership.transaction_items and filter out all
objects that are are not a subclass of MembershipType.

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> The Python SQL Toolkit and Object Relational Mapper
>
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[Jan Dalheimer]

I guess the simplest solutions are often the best… Turns out this actually works flawlessly (from what I’ve been able to tell so far), I didn’t expect SQLAlchemy to be smart enough to figure it out so I never even tried it this simple…

Thanks!

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