Get the category into the url of the product page I'm in the middle of learning Craft Commerce.  When I go to an individual product my URL says /shop/wella-shampoo/.  I'd like it to say /shop/**hair-care**/wella-shampoo/ because this is the category this product falls into.
I've tried adding shop/{categories.first().slug}/{slug} to the URL Format but I get the following error:

Internal Server Error
Craft\Commerce_ProductModel and its behaviors do not have a method or closure named &quot;categories&quot;.
It's a bit confusing, but since this product type has variants, using this should work:

shop/{object.categories.first().slug}/{object.slug}


If it didn't have variants, you wouldn't need the object reference in front of it.  The same thing applies if you're referencing a field inside of Matrix as well.

This has been made easier in the upcoming Commerce 2 so you don't have to worry about the differences.