Multiple database foreign key with Flask-SQLAchemy

example.py

import os

import flask
from flask import Flask
from flask.ext.cors import CORS
from flask.ext.sqlalchemy import SQLAlchemy

basedir = os.path.abspath(os.path.dirname(__file__))

app = Flask(__name__)

SQLALCHEMY_DATABASE_URI = 'sqlite:///' + os.path.join(basedir, 'main.db')

SQLALCHEMY_BINDS = {
        'otherDB': 'sqlite:///' + os.path.join(basedir, 'other.db')
}

app.config.from_object(__name__)

db = SQLAlchemy(app)
class User(db.Model):
    __bind_key__ = 'otherDB'

    id = db.Column(db.Integer, primary_key=True)
    name = db.Column(db.String)

class Car(db.Model):
    __bind_key__ = 'otherDB'  
    
    id = db.Column(db.Integer, primary_key=True)
    user_id = db.Column(db.Integer, db.ForeignKey(User.id))
    car_name = db.Column(db.String)

    user = db.relationship(User)


db.create_all()

@app.route('/create')
def create_user():
    user = User(name="meo")
    car = Car(user=user, car_name="Ferrari")
    db.session.add(user)
    db.session.add(car)
    db.session.commit()
    return "success"

@app.route('/get_car')
def get_car():
    cars = Car.query.all()
    res = []
    for car in cars:
        res.append({
            "car_id": car.id,
            "user_name": car.user.name
            })

    return flask.jsonify({"data": res})

app.run(debug=True)