/** * Calculates and returns the sum of all the numeric values that result from * applying a specified expression to each document in a group of documents * that share the same group by key. Ignores nun-numeric values. * * @see http://docs.mongodb.org/manual/reference/operator/aggregation/sum/ * @see Expr::sum * @param mixed|Expr $expression * @return Operator */ public function sum($expression) { $this->expr->sum($expression); return $this; }