onDestroying(model, options) {
        ghostBookshelf.Model.prototype.onDestroying.apply(this, arguments);

        return (options.transacting || ghostBookshelf.knex)('roles_users')
            .where('user_id', model.id)
            .del();
    }
@TODO:

The user model does not use bookshelf-relations yet.
Therefore we have to remove the relations manually.