Rethrowing exceptions under different names? What's the standard practice?

Question

Hi, currently I have this client code in my PHP MVC web app:

try {
  BookMapper::insert($book);
} catch (DbUniqueConstraintViolationException $e) {
  $errorList->addMessage($book . " already exists!");
}

I'm wondering is it bad practice to refer to the low-level framework Db* exceptions to my client code? If so, should I adjust my model code like so:

class BookAlreadyExistsException extends Exception { }

class BookMapper {
  public static function insert($book) {
    try {
      // call to DB-layer to insert $book
      // (not relevant to the question)
    } catch (DbUniqueConstraintViolationException $e) {
      throw new BookAlreadyExistsException();
    }
  }
}

and then use this new client-code...

try {
  BookMapper::insert($book);
} catch (BookAlreadyExistsException $e) {
  $errorList->addMessage($book . " already exists!");
}

Or something else? Or is the original method fine?

Thanks!

EDIT: Just want to add, the latter method reads the best IMO, but it comes with the object creation / rethrowing overhead and more significantly, it requires duplicating the rethrowing code in every mapper's insert() method. The former method is easy to implement and catch and works for any model but I remember reading somewhere that you shouldn't do it this way?

Answer 1

I think you should definitly throw your own exception.

But I would also consider a third option and that is letting the insert method return true for success and false for failure. Exceptions should be used for exceptions and the fact that a book already exists might actually be an expected/predictable case.

And if duplicate books are truly excetions that should not be possible (unless for programming errors), then you could as well stick with the database exception but in that case don't catch it. Let it bubble all the way up.

Answer 2

I highly recommend this article. Although it is written for Java, the principles are quite applicable to PHP as well. It has good guidelines for what types of exceptions you should be throwing and catching.