public function getTimestampHash($timestamp_field)
    {
        if (!$this->isReady()) {
            throw new LogicException('Collection is not ready');
        }

        $table_name = $this->getTableName();
        $conditions = $this->getConditions() ? " WHERE {$this->getConditions()}" : '';

        if ($this->count() > 0) {
            if ($join_expression = $this->getJoinExpression()) {
                return sha1($this->connection->executeFirstCell("SELECT GROUP_CONCAT($table_name.$timestamp_field ORDER BY $table_name.id SEPARATOR ',') AS 'timestamp_hash' FROM $table_name $join_expression $conditions"));
            } else {
                return sha1($this->connection->executeFirstCell("SELECT GROUP_CONCAT($table_name.$timestamp_field ORDER BY id SEPARATOR ',') AS 'timestamp_hash' FROM $table_name $conditions"));
            }
        }

        return sha1(get_class($this));
    }