code: def get_by_signature(user_id, app_id):
          '''
          Get the collection.
          '''
          try:
              return TabCollect.get(
                  (TabCollect.user_id == user_id) &
                  (TabCollect.post_id == app_id)
              )
          except:
              return Nonf get_by_signature(user_id, app_id):
          '''
          Get the collection.
          '''
          try:
              return TabCollect.get(
                  (TabCollect.user_id == user_id) &
                  (TabCollect.post_id == app_id)
              )
          except:
              return None
docs: Get the collection.