Deletes one instance

def delete(self):
        """ Deletes one instance """
        if self.instance is None:
            raise CQLEngineException("DML Query instance attribute is None")

        ds = DeleteStatement(self.column_family_name, timestamp=self._timestamp)
        for name, col in self.model._primary_keys.items():
            if (not col.partition_key) and (getattr(self.instance, name) is None): continue
            ds.add_where_clause(WhereClause(
                col.db_field_name,
                EqualsOperator(),
                col.to_database(getattr(self.instance, name))
            ))
        self._execute(ds)