public function ifNull($expression, $replacementExpression) : self
    {
        $this->expr->ifNull($expression, $replacementExpression);

        return $this;
    }
public function ifNull($expression, $replacementExpression) : self
    {
        $this->expr->ifNull($expression, $replacementExpression);

        return $this;
    }
public function ifNull ( $ expression , $ replacementExpression ) : self { $ this -> expr -> ifNull ( $ expression , $ replacementExpression ) ; return $ this ; }
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
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 .