/**
* Evaluates an expression and returns the value of the expression if the
* expression evaluates to a non-null value. If the expression evaluates to
* a null value, including instances of undefined values or missing fields,
* returns the value of the replacement expression.
*
* The arguments can be any valid expression.
*
* @see http://docs.mongodb.org/manual/reference/operator/aggregation/ifNull/
* @see Expr::ifNull
* @param mixed|Expr $expression
* @param mixed|Expr $replacementExpression
* @return self
*/
public function ifNull($expression, $replacementExpression)
{
$this->expr->ifNull($expression, $replacementExpression);
return $this;
}