/** * Takes two sets and returns an array containing the elements that only * exist in the first set. * * The arguments can be any valid expression as long as they each resolve to an array. * * @see http://docs.mongodb.org/manual/reference/operator/aggregation/setDifference/ * @see Expr::setDifference * @param mixed|Expr $expression1 * @param mixed|Expr $expression2 * @return self */ public function setDifference($expression1, $expression2) { $this->expr->setDifference($expression1, $expression2); return $this; }