code: def get_by_signature(user_id, app_id):
          '''
          get by user ID, and app ID.
          '''
          try:
              return TabEvaluation.get(
                  (TabEvaluation.user_id == user_id) & (TabEvaluation.post_id == app_id)
              )
          except:
              return Nonf get_by_signature(user_id, app_id):
          '''
          get by user ID, and app ID.
          '''
          try:
              return TabEvaluation.get(
                  (TabEvaluation.user_id == user_id) & (TabEvaluation.post_id == app_id)
              )
          except:
              return None
docs: get by user ID, and app ID.