Flask-SQLAlchemy One to Many relationship failed to locate a name

Question

I have a Flask app with Flask-SQLAlchemy. I'm trying to make a one to many relationship from a user to a trip. I saw that, if I use the declarative_base of SQLAlchemy it works but when I use the db.Model from Flask-SQLAlchemy it dosen't work.

I'm getting the following error:

sqlalchemy.exc.InvalidRequestError: When initializing mapper Mapper|User|User, expression 'Trip' failed to locate a name ("name 'Trip' is not defined"). If this is a class name, consider adding this relationship() to the <class 'models.user.User'> class after both dependent classes have been defined.

Here are the models:

database.py

from flask import abort 
from flask_sqlalchemy import SQLAlchemy 
from sqlalchemy import exc

db = SQLAlchemy()


class Mixin(db.Model):
    __abstract__ = True
    id = db.Column(db.Integer, primary_key=True)
    date = db.Column(db.DateTime, default=db.func.now())
    date_changed = db.Column(
        db.DateTime, nullable=True, onupdate=db.func.now())
    date_to = db.Column(db.DateTime, nullable=True)

user.py

from database import Mixin, db

class User(Mixin, db.Model):
    __tablename__ = 'User'        
    trips = db.relationship(
        'Trip',
        backref=db.backref('user', lazy='joined'),
        lazy='dynamic'
    )     

trip.py

from database import Mixin, db

class Trip(Mixin, db.Model):
    __tablename__ = 'Trip'
    title = db.Column(db.String(100), nullable=False)
    description = db.Column(db.String(250), nullable=False)        
    user_id = db.Column(
        db.Integer,
        db.ForeignKey('User.id'),
        nullable=False
    )       

Answer 1

Storing all models in one file is a good idea, but only for a small number of models. For the large I would prefer to store models in semantically separate files.

models
├── __init__.py
├── model1.py
├── model2.py
└── model3.py

In the __init__.py need to import all models files

import models.model1.py
import models.model2.py
import models.model3.py