Laravel 5: How to use firstOrNew with additional relationship fields

I have a CMS that allows the user to save and create bike tours. Each bike tour also has categories, which are definined using Laravel's Many to Many relationship utilising an intermediary pivot table. At the point of saving a tour, we don't know if the tour is an existing one being edited, or a new one.

I think I should be using Laravel's firstOrNew method for saving the tour, and the sync method for saving categories. However, all the tutorials very simplistically just give the example of passing a single object to the function like so:

$tour = Tour::firstOrNew($attributes);

But what happens when my $attributes also contains extra stuff, like the categories which are linked to a relationship table, and which I will need to save in the next step? For example this very good tutorial gives the following example:

$categories = [7, 12, 52, 77];

$tour = Tour::find(2);

$tour->categories()->sync($categories);

But what happens if the category data is bundled with the data for the rest of the tour, and instead of using find I need to use firstOrNew to create the tour? Should I keep the categories in the $attributes while I instantiate the tour, then run the sync, then unset them before saving the tour, or...? Is there a better way to achieve this?

via Chebli Mohamed