Flask sqlalchemy, how to avoid circular dependency in this case? - database

Question

I'v been banging my head over this for 2 days straight and i still can understand/find a way to do this and it looks super simple but im obviously overlooking something (also im fairly new to DBs :) ).
I want to have Owner and Pet model.
Pets have 'owner ids' as foreign keys, and Owners have 'pets' as relationship, so far so good.
But now i also want Owners to have one 'pet id' written as 'favorite pet'.
Having foreign keys in both models (each others keys) started to make bunch of different problems (different depending on how i try to solve it, but either circular dependency or some multipath error)
I also noticed that if i avoid having 'favourite_pet_id'-foreign key in Owner model, keeping only favourite_pet-relationship, then i dont have this written anywhere in DB (at least not visible), it exists only as 'relationship' ?
What would be correct way of doing this ?
Thanks in advance !
from flask import Flask
from flask_sqlalchemy import SQLAlchemy
app = Flask(__name__)
app.config['SQLALCHEMY_DATABASE_URI'] = 'sqlite://'
app.config['SQLALCHEMY_TRACK_MODIFICATIONS'] = False
db = SQLAlchemy(app)
class Owner(db.Model):
id = db.Column(db.Integer, primary_key=True)
pets = db.relationship('Pet', foreign_keys='Pet.owner_id')
favourite_id = db.Column(db.Integer, db.ForeignKey('pet.id'))
favourite = db.relationship('Pet', uselist=False, foreign_keys='Owner.favourite_id')
class Pet(db.Model):
id =db.Column(db.Integer, primary_key=True)
owner_id = db.Column(db.Integer, db.ForeignKey('owner.id'))
owner = db.relationship('Owner', uselist=False, back_populates='pets', foreign_keys='Pet.owner_id')
o = Owner() # one owner
p1 = Pet() # pet 1
p2 = Pet() # pet 2
p1.owner=o # setting owner for pet1
p2.owner=o # setting owner for pet2
o.favourite=p2 # setting pet2 to be favourite
#db.session.add(o)
#db.session.add(p1)
#db.session.add(p2)
#db.session.commit()
print (p1.owner) # owner
print (p2.owner) # owner
print (p1) # pet 1
print (p2) # pet 2
print (o.pets) # owners pets
print (o.favourite) # favourite pet

Answer 1

Here's a working version of your code below. The key is being more explicit about the relationship (e.g. join condition) as there are multiple relationships between the models/tables.
Note: While not part of your question/request, I also reformatted a bit for PEP8 conformity and readability. The later is a good practice to adopt early on as model files typically grow quickly and can become very difficult to read, digest, and debug.
from flask import Flask
from flask_sqlalchemy import SQLAlchemy
app = Flask(__name__)
app.config['SQLALCHEMY_DATABASE_URI'] = 'sqlite:///:memory:'
app.config['SQLALCHEMY_TRACK_MODIFICATIONS'] = False
db = SQLAlchemy(app)
class Owner(db.Model):
id = db.Column(
db.Integer,
primary_key=True,
)
favourite_pet_id = db.Column(
db.Integer,
db.ForeignKey('pet.id'),
nullable=True,
)
favourite_pet = db.relationship(
'Pet',
uselist=False,
foreign_keys=[favourite_pet_id],
primaryjoin='Pet.owner_id == Owner.favourite_pet_id',
)
class Pet(db.Model):
id = db.Column(
db.Integer,
primary_key=True,
)
owner_id = db.Column(
db.Integer,
db.ForeignKey('owner.id'),
)
owner = db.relationship(
'Owner',
uselist=False,
foreign_keys=[owner_id],
primaryjoin='Pet.owner_id == Owner.id',
backref=db.backref(
'pets',
uselist=True,
),
)
db.create_all()
o = Owner() # one owner
p1 = Pet() # pet 1
p2 = Pet() # pet 2
p1.owner = o # setting owner for pet1
p2.owner = o # setting owner for pet2
o.favourite_pet = p2 # setting pet2 to be favourite
print(p1.owner) # owner
print(p2.owner) # owner
print(p1) # pet 1
print(p2) # pet 2
print(o.pets) # owners pets
print(o.favourite) # favourite pet