code: def add_sort(self, field, ascending=True):
          
          # No-op on blank field
          if not field:
              return self
          self._add_sort(field, ascending=ascending)
          return self
docs: Sort the search results by a certain field.
  
          If this method is called multiple times, the later sort fields are given lower priority,
          and will only be considered when the eariler fields have the same value.
  
          Arguments:
              field (str): The field to sort by.
                      The field must be namespaced according to Elasticsearch rules
                      using the dot syntax.
                      For example, ``"mdf.source_name"`` is the ``source_name`` field
                      of the ``mdf`` dictionary.
              ascending (bool): If ``True``, the results will be sorted in ascending order.
                      If ``False``, the results will be sorted in descending order.
                      **Default**: ``True``.
          Returns:
              SearchHelper: Self