Collecting Model Statistics

I have a model hierarchy that looks like:

User has many days, a day has many items.

I'm trying to collect cheap (processing) statistics about the items,
specifically on the count of duplicates.

Ultimately I want to know if an item is such that it is included in
many days, and further more to know the rank of top items in the

Currently I have a Count function that determines how many times an
Item is found, but it's expensive to execute when it's run for
thousands of items. Any ideas?