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.
>
> 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'
> }
>
> --
> SQLAlchemy -
> The Python SQL Toolkit and Object Relational Mapper
>
> http://www.sqlalchemy.org/
>
> To post example code, please provide an MCVE: Minimal, Complete, and
> Verifiable Example. See http://stackoverflow.com/help/mcve for a full
> description.
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--
SQLAlchemy -
The Python SQL Toolkit and Object Relational Mapper
http://www.sqlalchemy.org/
To post example code, please provide an MCVE: Minimal, Complete, and Verifiable
Example. See http://stackoverflow.com/help/mcve for a full description.
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