Undefined join condition between parent/child tables - join

Question

Here's a model for a basic todo app, in which a user can create a tasklist that contains tasks:
from flask.ext.sqlalchemy import SQLAlchemy
from allot import db
class User(db.Model):
id = db.Column(db.Integer, primary_key=True)
username = db.Column(db.String)
tasklists = db.relationship('Tasklist', backref='user', lazy='dynamic')
tasks = db.relationship('Task', backref='user', lazy='dynamic')
def __init__(self, username, email):
self.username = username
class Tasklist(db.Model):
id = db.Column(db.Integer, primary_key=True)
title = db.Column(db.String)
user_id = db.Column(db.Integer, db.ForeignKey('User.id'))
tasks = db.relationship('Task', backref='tasklist', lazy='dynamic')
def __init__(self, title, description, user_id):
self.title = title
self.user_id = user_id
class Task(db.Model):
id = db.Column(db.Integer, primary_key=True)
title= db.Column(db.String)
user_id = db.Column(db.Integer, db.ForeignKey('User.id'))
tasklist_id = db.Column(db.Integer, db.ForeignKey('Tasklist.id'))
def __init__ (self, title, status, user_id):
self.title = title
self.user_id = user_id
self.tasklist_id = tasklist_id
It generates this error:
ArgumentError: Could not determine join condition between parent/child
tables on relationship User.tasklists. Specify a 'primaryjoin'
expression. If 'secondary' is present, 'secondaryjoin' is needed as
well.
Adding primaryjoin arguments in the relationships, like this:
tasklists = db.relationship('Tasklist', backref='user', primaryjoin='user.id==Tasklist.user_id', lazy='dynamic')
... returns:
InvalidRequestError: One or more mappers failed to initialize - can't
proceed with initialization of other mappers.
What's wrong with the model?
I searched for an explanation in the [relationship][1] doc and on SO but to no avail.

Answer 1

One possibility is that when you're referencing foreign keys your using the uppercase "User" when you actually need to use the lowercase tablename reference. Try this:
from flask.ext.sqlalchemy import SQLAlchemy
from allot import db
class User(db.Model):
__tablename__ = 'user'
id = db.Column(db.Integer, primary_key=True)
username = db.Column(db.String)
tasklists = db.relationship('Tasklist', backref='user', lazy='dynamic')
tasks = db.relationship('Task', backref='user', lazy='joined'))
def __init__(self, username, email):
self.username = username
class Tasklist(db.Model):
__tablename__ = 'tasklist'
id = db.Column(db.Integer, primary_key=True)
title = db.Column(db.String)
user_id = db.Column(db.Integer, db.ForeignKey('user.id'))
tasks = db.relationship('Task', backref='tasklist', lazy='dynamic')
def __init__(self, title, description, user_id):
self.title = title
self.user_id = user_id
class Task(db.Model):
__tablename__ = 'task'
id = db.Column(db.Integer, primary_key=True)
title= db.Column(db.String)
user_id = db.Column(db.Integer, db.ForeignKey('user.id'))
tasklist_id = db.Column(db.Integer, db.ForeignKey('tasklist.id'))
def __init__ (self, title, status, user_id):
self.title = title
self.user_id = user_id
self.tasklist_id = tasklist_id