Question

I am trying to re-architect a web application I developed to use the MVC pattern, but I’m not sure how to handle errors. For example:

class AM_Products extends AM_Object 
{
    public function save( $new_data = array() ) 
    {
        // Validate input

        // Save input
    }
}

If I pass invalid input to save, should I throw an exception like this:

class AM_Products extends AM_Object 
{
    public function save( $new_data = array() ) 
    {
        // Validate input
        if ( ! validate( 'text', $new_data['name'] ) ) {
            throw new Exception( 'Invalid data entered' );
        }

        // Save input
    }
}

Or instead, should I add an extra function and leave it to the view/controller:

if ( $product->save( $data )->has_error() ) {
    $error = $product->get_error();
}

echo '<p>' . $error . '</p>';

Answer 1

Don’t throw an exception. Exceptions are for exceptional situations – invalid data entered into a form should not trigger an exception.

Your model should have some sort of error state set, either on the model itself or on the individual fields. The post-back should “fall through” and display the same form that was originally shown, with error messages and/or highlighted fields indicating where the error is so the user can fix it.

Throwing exceptions for validation is going to lead to a very fragile and difficult to use system. What happens if you want to simply show the user that one of the fields they supplied is invalid and give them a chance to correct it? How are you going to catch an exception and know how to display the associated record/form?