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?
I have also posted this question in Stack Overflow
<https://stackoverflow.com/questions/50031376/override-relationship-behaviour-in-sqlalchemy>,
in case it means something.
Best!
--
SQLAlchemy -
The Python SQL Toolkit and Object Relational Mapper
http://www.sqlalchemy.org/
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