code: def _delete_null_columns(self, conditionals=None):
          
          ds = DeleteStatement(self.column_family_name, conditionals=conditionals, if_exists=self._if_exists)
          deleted_fields = False
          static_only = True
          for _, v in self.instance._values.items():
              col = v.column
              if v.deleted:
                  ds.add_field(col.db_field_name)
                  deleted_fields = True
                  static_only &= col.static
              elif isinstance(col, columns.Map):
                  uc = MapDeleteClause(col.db_field_name, v.value, v.previous_value)
                  if uc.get_context_size() > 0:
                      ds.add_field(uc)
                      deleted_fields = True
                      static_only |= col.static
  
          if deleted_fields:
              keys = self.model._partition_keys if static_only else self.model._primary_keys
              for name, col in keys.items():
                  ds.add_where(col, EqualsOperator(), getattr(self.instance, name))
              self._execute(ds)
docs: executes a delete query to remove columns that have changed to null