SQLAlchemy DetachedInstanceError when Unit testing with Flask app.test_client

Question

My goal is to be able to test routes with the Flask app test_client using self.client.post(url, .... The problem that I'm running into is any time I make a request using it, I hit a DetachedInstanceError from SQLAlchemy, saying that either refresh or lazy loading operations cannot occur.

Here's my setup:

class BaseTestCase(unittest.TestCase):

   def setUp(self):
      self.postgresql = Postgresql()
      config = {
         'FLASK_ENV': 'test',
         'SQLALCHEMY_DATABASE_URI': self.postgresql.url()
      }
      with patch.dict('os.environ', config):
         self.app = create_app()
         self.client = self.app.test_client()
         ctx = self.app.app_context()
         ctx.push()
         db.create_all()
         self.assertEqual(app.testing, True)
      reset_fake_data()

   def tearDown(self):
      db.session.remove()
      self.postgresql.stop()

   def post(self, url, data={}, headers={}):
      headers = {'Content-Type': 'application/json', **headers}
      return self.client.post(url, headers=headers, data=json.dumps(data))

   def post_with_auth_token(self, url, data={}, headers={}):
      headers = {'Authorization': f'Bearer {self.auth_token}'}
      return self.post(url, headers=headers, data=data)

Then in another file, I'm calling this with response = self.post('/api/users/register', data=data) and getting the error:

sqlalchemy.orm.exc.DetachedInstanceError: Parent instance  is not bound to a Session; lazy load operation of attribute 'courses' cannot proceed (Background on this error at: http://sqlalche.me/e/bhk3)

Below is the route. I create a user object and then attempt to add a course to it but apparently, when I try to do user.courses.append, it's unable to lazy load the courses. I've looked into disabling the expire_on_commit attribute, but I'm not sure how to do that only in the cases of testing, or if there's a better way to go about this.

I read that converting from lazy to eager loading could also fix it, but I'd rather keep things loading lazily and it didn't seem to fix it when I set the relationship to be lazy='subquery' anyways.

@users.route('/register', methods=['POST'])
def register():
   data = request.get_json()
   try:
      user = User.from_data(**data)
   except UnprocessableEntity:
      return jsonify(msg='Missing parameters'), 422
   except Conflict:
      return jsonify(msg='Email already exists'), 409
   except Unauthorized:
      return jsonify(msg='Unauthorized'), 401
   if data.get('invitation_token'):
      invite = CourseInvite.query.filter_by(token=data['invitation_token']).first_or_404('Invite not found')
      if invite.email and invite.email != user.email:
         return jsonify(msg='Email mismatch'), 401
      elif invite.used:
         return jsonify(msg='Invite already used'), 401
      user.courses.append(CourseEnrollment(course_id=invite.course.id))
      invite.used = True
   db.session.add(user)
   db.session.commit()
   return jsonify(msg='User successfully created'), 201

SQLAlchemy DetachedInstanceError when Unit testing with Flask app.test_client

0 Answers