Laravel 5.1 - How to get not attached pivot registers for add

I am doing a CRUD with Laravel 5.1. When I click the add button, a form with a select box Is shown (the controller sends all the data correctly). All works fine, but I want to improve the form not showing elements attached to the model.

Example: 2 models, User and Role, with a Relationship Many to Many. My controller gets all the roles attachedto the current user EXCEPT which are already attached.

Any idea how to get this?

via Chebli Mohamed