/**
* 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.
*
* @return Operator
*/
public function sum($expression)
{
$this->expr->sum($expression);
return $this;
}
/**
* Returns the year portion of a date.
*
* The argument can be any expression as long as it resolves to a date.
*
* @see http://docs.mongodb.org/manual/reference/operator/aggregation/year/
* @see Expr::year
* @param mixed|Expr $expression
* @return self
*/
public function year($expression)
{
$this->expr->year($expression);
return $this;
}