code: def set_field(self, field_name, field_val=None):
          
          field_name = self._normalize_field_name(field_name)
          self.fields_set.append(field_name, [field_name, field_val])
          return self
docs: set a field into .fields attribute
  
          n insert/update queries, these are the fields that will be inserted/updated into the db