protected function _buildUpdate()
        {
            $query = [];
            $query[] = "UPDATE {$this->_quoteIdentifier($this->_table_name)} SET";

            $field_list = [];

            foreach ($this->_dirty_fields as $key => $value) {
                if(!Arrays::exists($key, $this->_expr_fields)) {
                    $value = '?';
                }

                $field_list[] = "{$this->_quoteIdentifier($key)} = $value";
            }

            $query[] = join(", ", $field_list);
            $this->_addIdColumnConditions($query);

            return join(" ", $query);
        }