flask-restless: POST method fails when called first

requirements.txt

Flask
SQLAlchemy
Flask-Restless

testapp.py

from flask import Flask
from flask.ext.restless import APIManager

from sqlalchemy import create_engine, Column, Integer, String
from sqlalchemy.orm import scoped_session, create_session, relationship
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.schema import ForeignKey

# lazy SQLAlchemy setup (http://flask.pocoo.org/snippets/22/)
engine = None
db_session = scoped_session(lambda: create_session(bind=engine,autocommit=False,autoflush=False))

def init_engine(uri, **kwargs):
    global engine
    engine = create_engine(uri, **kwargs)
    return engine

Base = declarative_base()
Base.query = db_session.query_property()

class UserVoting(Base):
    __tablename__ = 'user_voting'

    id = Column(Integer, primary_key=True)
    vote = Column(String(1), nullable=False)
    user_id = Column(Integer, ForeignKey('app_user.id'), nullable=False)
    voting_review_id = Column(Integer, ForeignKey('voting_review.id'), nullable=False)

class VotingReview(Base):
    __tablename__ = 'voting_review'

    id = Column(Integer, primary_key=True)
    title = Column(String(160), nullable=False)
    user_id = Column(Integer, ForeignKey('app_user.id'), nullable=False)

    userVotings = relationship('UserVoting', backref='votingReview')

class User(Base):
    __tablename__ = 'app_user'

    id = Column(Integer, primary_key=True)
    name = Column(String(255))

    votingReviews = relationship('VotingReview', backref='user')
    userVotings = relationship('UserVoting', backref='user')

app = Flask(__name__)

# Create the Flask-Restless API manager.
manager = APIManager(app, session=db_session)
manager.create_api(VotingReview, methods=['GET', 'POST', 'DELETE'])

@app.teardown_request
def shutdown_session(exception=None):
    db_session.remove()

if __name__ == '__main__':
    init_engine(app.config['DATABASE_URL'])
    app.run(host='0.0.0.0')

testapp_tests.py

import os
import unittest
import tempfile
import json
from testapp import app, init_engine

class TestappTestCase(unittest.TestCase):

    def setUp(self):
        self.db_fd, app.config['DATABASE'] = tempfile.mkstemp()
        database_url = 'sqlite:///%s' % self.db_fd
        init_engine(database_url)
        app.config['TESTING'] = True
        self.app = app.test_client()

    def tearDown(self):
        os.close(self.db_fd)
        os.unlink(app.config['DATABASE'])

    def test_root(self):
        response = self.app.post('/api/voting_review',
                                 data=json.dumps({'title': u'title', 'voting_id': 23}))
        self.assertEqual(response.status_code, 200)

if __name__ == '__main__':
    unittest.main()