On Thu, Apr 26, 2018 at 11:04 AM, Diego Quintana <daquinta...@gmail.com> wrote:
> Hello.
>
> Say I have three tables in a declarative fashion, `Parent`, `Child`, and
> `Pet`, in such way that
>
> * `Parent` has a many-to-many relationship with both `Child` and `Pet`,
> meaning that a Parent can own a Child and its pets, and also a Pet without
> its Child.
> * `Child` has a one-to-many relationship with `Pet`
>
> The code for them is (using Flask-SQLAlchemy, although I believe the
> solution lives in the realm of SQLAlchemy rather than in Flask).
>
> class Parent(db.Model):
> __tablename__ = 'parents'
>
> id = db.Column(db.Integer, primary_key=True)
> name = db.Column(db.String(64))
>
> # many to many relationship between parent and children
> # my case allows for a children to have many parents. Don't ask.
> children = db.relationship('Child',
> secondary=parents_children_relationship,
> backref=db.backref('parents',
> lazy='dynamic'),
> lazy='dynamic')
>
> # many to many relationship between parents and pets
> pets = db.relationship('Pet',
> secondary=users_pets_relationship,
> backref=db.backref('parents',
> lazy='dynamic'), #
> lazy='dynamic')
>
> # many to many relationship between parents and children
> parents_children_relationship =
> db.Table('parents_children_relationship',
> db.Column('parent_id', db.Integer, db.ForeignKey('parents.id')),
> db.Column('child_id', db.Integer, db.ForeignKey('children.id')),
> UniqueConstraint('parent_id', 'child_id'))
>
> # many to many relationship between User and Pet
> users_pets_relationship = db.Table('users_pets_relationship',
> db.Column('parent_id', db.Integer, db.ForeignKey('parents.id')),
> db.Column('pet_id', db.Integer, db.ForeignKey('pets.id')),
> UniqueConstraint('parent_id', 'pet_id'))
>
> class Child(db.Model):
> __tablename__ = 'children'
> id = db.Column(db.Integer, primary_key=True)
> name = db.Column(db.String(64))
> # parents = <backref relationship with User model>
>
> # one to many relationship with pets
> pets = db.relationship('Pet', backref='child', lazy='dynamic')
>
>
> class Pet(db.Model):
> __tablename__ = 'pets'
> id = db.Column(db.Integer, primary_key=True)
> name = db.Column(db.String(64))
> # child = backref relationship with cities
> child_id = db.Column(db.Integer, db.ForeignKey('children.id'),
> nullable=True)
> # parents = <relationship backref from User>
>
>
>
> I would like to do something like this
>
> parent_a = Parent()
> child_a = Child()
> pet_a = Pet()
>
>
>
> I can then do this
>
> parent_a.children.append(child_a)
> # commit/persist data
> parent_a.children.all() # [child_a]
>
>
>
> I would like to achieve something like this
>
> child_a.pets.append(pet_a)
> parent_a.children.append(child_a)
> # commit/persist data
> parent_a.children.all() # [child_a]
> parent_a.pets.all() # [pet_a], because pet_a gets
> # automatically added to parent using some sorcery
> # like for child in parent_a.children.all():
> # parent.pets.append(child.pets.all())
> # or something like that.
>
> I can achieve this with a method in the Parent object like
> add_child_and_its_pets(), but I would like to override the way relationship
> works, so I don't need to override other modules that may benefit from this
> behaviour, like Flask-Admin for instance.
>
>
> Basically how should I override the backref.append method or the
> relationship.append method to also append other objects from other
> relationships at call time i.e. on the python side? How should I override
> the remove methods as well?
this seems like coordination of two separate relationships, so use
attribute events for that,
http://docs.sqlalchemy.org/en/latest/orm/events.html?highlight=attributeevent#sqlalchemy.orm.events.AttributeEvents
:
@event.listens_for(Parent.children, "append")
def _append_pets(parent, child, initiator):
parent.pets.extend(child.pets) # or whatever it is you need
you would need to look at the "append" and "remove" events.
>
>
> I have also posted this question in Stack Overflow, in case it means
> something.
>
>
> Best!
>
>
>
> --
> 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
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--
SQLAlchemy -
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http://www.sqlalchemy.org/
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