title: Get result by id fails
body: <p>I'm trying to use celery in my Flask-based web app and monitore it's state. My idea is to store task's id in session, and use it for state polling.</p>
  
  <p>Here is content of my tasks.py</p>
  
  <pre><code>from traceback import format_exc
  import settings
  from celery import Celery, current_task
  from celery.utils.log import get_task_logger
  from celery.result import AsyncResult
  
  from nimble_auth import make_nimble_req
  
  logger = get_task_logger(__name__)
  
  celery = Celery('tasks')
  celery.config_from_object(settings)
  
  @celery.task
  def make_work(param1, param2):
      try:
          # work here
          current_task.update_state(state='PROGRESS', meta={'current': page, 'total': total_pages})
      except StandardError:
          logger.error(format_exc())
  
  def get_asynctask(task_id):
      return AsyncResult(task_id)
  </code></pre>
  
  <p>Except from main app's file web.py</p>
  
  <pre><code>@app.route('/task_state/&lt;task_id&gt;')
  def task_state(task_id):
      return get_asynctask(task_id).state # from task.py
  
  @app.route('/reset_task')
  def reset_task():
      del session['celery_task_id']
      return redirect(url_for('index'))
  
  @app.route('/run')
  def run_task():
      task = make_work.delay(1, 2)
      session['celery_task_id'] = task.task_id
      flash('Task submitted OK!')
  
      return redirect(url_for('index'))
  </code></pre>
  
  <p>Settings file:</p>
  
  <pre><code>BROKER_URL = 'mongodb://localhost:27017/dramba'
  CELERY_RESULT_BACKEND = "mongodb"
  CELERY_RESULT_DBURI = 'mongodb://localhost:27017/dramba'
  </code></pre>
  
  <p>But when I'm trying to get task's state, I'm getting NotImplementedError: No result backend configured. </p>
  
  <p>Full stacktrace:</p>
  
  <pre><code>Traceback (most recent call last):
    File "/Users/cleg/Projects/Venvs/Dramba/lib/python2.7/site-packages/flask/app.py", line 1701, in __call__
      return self.wsgi_app(environ, start_response)
    File "/Users/cleg/Projects/Venvs/Dramba/lib/python2.7/site-packages/flask/app.py", line 1689, in wsgi_app
      response = self.make_response(self.handle_exception(e))
    File "/Users/cleg/Projects/Venvs/Dramba/lib/python2.7/site-packages/flask/app.py", line 1687, in wsgi_app
      response = self.full_dispatch_request()
    File "/Users/cleg/Projects/Venvs/Dramba/lib/python2.7/site-packages/flask/app.py", line 1360, in full_dispatch_request
      rv = self.handle_user_exception(e)
    File "/Users/cleg/Projects/Venvs/Dramba/lib/python2.7/site-packages/flask/app.py", line 1358, in full_dispatch_request
      rv = self.dispatch_request()
    File "/Users/cleg/Projects/Venvs/Dramba/lib/python2.7/site-packages/flask/app.py", line 1344, in dispatch_request
      return self.view_functions[rule.endpoint](**req.view_args)
    File "/Users/cleg/Projects/GitHub/Dramba/dramba.py", line 13, in task_state
      return get_asynctask(task_id).state
    File "/Users/cleg/Projects/Venvs/Dramba/lib/python2.7/site-packages/celery/result.py", line 284, in state
      return self.backend.get_status(self.id)
    File "/Users/cleg/Projects/Venvs/Dramba/lib/python2.7/site-packages/celery/backends/base.py", line 490, in _is_disabled
      raise NotImplementedError('No result backend configured.  '
  NotImplementedError: No result backend configured.  Please see the documentation for more information.
  </code></pre>
  
  <p>What am I doing wrong? </p>