带有 order_by 的 Flask SQLAlchemy 查询返回错误没有这样的列

Question

Here is my models:

from flask import Flask
from flask_sqlalchemy import SQLAlchemy


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


class Person(db.Model):
    __tablename__ = 'persons'
    id = db.Column(db.Integer, primary_key=True)
    name = db.Column(db.String(64), nullable=False, unique=True)
    pets = db.relationship('Pet', backref='owner', lazy='dynamic')

    def __init__(self, *args, **kwargs):
        super(Person, self).__init__(*args, **kwargs)

    def __repr__(self):
        return f'<Person id:{self.id} name:{self.name}>'

class Pet(db.Model):
    __tablename__ = 'pets'
    id = db.Column(db.Integer, primary_key=True)
    name = db.Column(db.String(64), nullable=False, unique=True)
    owner_id = db.Column(db.Integer, db.ForeignKey('persons.id'), nullable=False)

    def __init__(self, *args, **kwargs):
        super(Pet, self).__init__(*args, **kwargs)

    def __repr__(self):
        return f'<Pet id:{self.id} name:{self.name} owner_id:{self.owner_id}>'

I`m trying to get persons list ordered by count of pets with this query:

persons = Person.query.order_by(Person.pets).all()

And getting this error:

sqlalchemy.exc.OperationalError: (sqlite3.OperationalError) no such column: pets.owner_id
[SQL: SELECT persons.id AS persons_id, persons.name AS persons_name
FROM persons ORDER BY persons.id = pets.owner_id]

What I`m doing wrong? I have a suggestion that I need to make a request with join(), but research in google did not give working solutions.

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.