Flask SQLAlchemy query with order_by returns error no such column

Question

Here is my models:

from flask import Flask
from flask_sqlalchemy import SQLAlchemy


app = Flask(__name__)
db = SQLAlchemy(app)


class Person(db.Model):
    __t         


        

Answer 1

If you inspect the SQL, you'll note that there is no FROM item pets, and so ORDER BY persons.id = pets.owner_id fails. This happens because the relationship attribute Person.pets renders as its ON clause in query context, or in other words as persons.id = pets.owner_id. There are many ways to form the proper query, for example using a JOIN and GROUP BY:

Person.query.\
    outerjoin(Person.pets).\
    group_by(Person.id).\
    order_by(func.count(Pet.id)).\
    all()

The LEFT OUTER JOIN ensures that persons without pets are considered as well.