code: def _filter_contains(self, term, field_name, field_type, is_not):
          
          if field_type == 'text':
              term_list = term.split()
          else:
              term_list = [term]
  
          query = self._or_query(term_list, field_name, field_type)
          if is_not:
              return xapian.Query(xapian.Query.OP_AND_NOT, self._all_query(), query)
          else:
              return query
docs: Splits the sentence in terms and join them with OR,
          using stemmed and un-stemmed.
  
          Assumes term is not a list.