1 - Create a fetch request for your desired entity.
2 - Create a sort descriptor for the key whose max value you're
interested in.
3 - Set the fetch request's sort descriptor to the above.
4 - Create any predicates needed for filtration (ie, whatever you
may or may not have used in the tree controller).
5 - Set the fetch request's predicate to the above if any.
6 - Execute the fetch request and get the last (or first) object of
the results (checking for errors, minding the set-versus-array
gotchas, etc.).
Wow, really? No way to just get at all the items in the tree
controller with some key path, then use @max? It seems that this would
be possible with an NSArray:
