Does nested-sets model suit only for categories/catalogs (e.g fruits, vegetables), or for end-elements too (such as apple, onion etc.)?
Let's imagine my website is selling fruits and vegetables, and it has categories of products like:
Products -> Fruits -> Apple, Orange, Grape
Products -> Vegetables -> Onion, Carrot
Apple, Orange, Grape, Onion, Carrot - are not categories of anything so they have no children (and won't have ever).
The question is should I include Apple, Orange, Grape, Onion, Carrot in my Nested-sets table or in separate one with link to its category_id? (e.g. the scheme of table of products would be like [id, name, price, category_id]).
