How to define an accessor for an existing field in a model in Laravel 5?

I have a model called Book for a table called books, that has a field called cover_image.

However, I want to define an accessor for the field cover_image instead of just retrieving the default value.

This is how I attempted to do this:

class Book extends Model {

    public function getCoverImageAttribute() {

         if ($this->cover_image === null) { // Not sure how to check current value?
               return "a.jpg"
         }
         return $this->cover_image; 
    } 

}

However the above of course does not work for two reasons:

• Calling $this->cover_image again would cause recursion
• Laravel dosen't like that I created an accessor for an existing field in the database

How can I fix this?

via Chebli Mohamed