new_contents: @endpoints.method(message_types.VoidMessage, ConferenceForms,
          path='filterPlayground',
          http_method='GET', name='filterPlayground')
  def filterPlayground(self, request):
      q = Conference.query()
      # simple filter usage:
      # q = q.filter(Conference.city == "Paris")
  
      # advanced filter building and usage
      # field = "city"
      # operator = "=="
      # value = "London"
      # f = ndb.query.FilterNode(field, operator, value)
      # q = q.filter(f)
  
      # TODO
      # add 2 filters:
      # 1: city equals to London
      # 2: topic equals "Medical Innovations"
  
      return ConferenceForms(
          items=[self._copyConferenceToForm(conf, "") for conf in q]
      )
subject: add TODO_4 - simplePlayground to Lesson 4