iOS核心数据：NSPredicate筛选具有属性最大值的对象

Question

I have Members table, for example with the following entries:

Name | Group | JoinDate
Member1 | Group1 | 2011-03-12
Member2 | Group2 | 2011-03-10
Member3 | Group1 | 2011-03-13
Member4 | Group3 | 2011-03-9
Member5 | Group2 | 2011-03-23

I want to fetch the most recent member from each group using NSFetchedResultController. In the above case, the result will be:

Name | Group | JoinDate
Member3 | Group1 | 2011-03-13
Member5 | Group2 | 2011-03-23
Member4 | Group3 | 2011-03-09

I was thinking maybe it would involve some complex NSPredicate. Any idea how to do this? Thanks!

Answer 1

Finding the most recent in each group all in one operation seems difficult, so break it down. It's easy to find the most recent member in a single group, so iterate over the set of groups and pick the most recent member out of each group.

If you have a Group entity that's related to Member, so that you can fetch the sets of members in each group, then you could use KVC's '@max' collection operator to easily select the most recent join date from each of those sets.