CoreData most recent entries in group

Question

I'm wondering if there's a (simple) way to achieve the following using a NSFetchedResultsController:

say I have a visitor log table like this:

visitor_name (String)
visit_time (Date)
visited_building_id (String)

example data:

John, 2015-09-25 10:00:00, ABC
Jane, 2015-09-25 10:20:00, ABC
Mark, 2015-09-25 10:40:00, ABC
Jane, 2015-09-25 10:10:00, DEF

And I want to show a table view which shows who most recently visited each building, so for the data above it should show two rows:

[ABC, Mark, 9/25 10:40]
[DEF, Jane, 9/25 10:10]

Answer 1

One solution would be to:

1. Sort the entries by visited_building_id (ascending or descending as you prefer) and then by visit_time (descending).
2. Specify visited_building_id as the sectionNameKeyPath for the fetched results controller. The FRC will therefore have a section for each building, and the first object in each section will be the most recent visit. You want your table view rows to display the first item in each section of the FRC. So...

3. Amend the tablewView datasource methods to remap the FRC sections to your tableView's rows. So for example numberOfRowsInSection would return frc.sections.count . cellForRowAtIndexPath would construct a new NSIndexPath using

   NSIndexPath *newIndexPath = [NSIndexPath indexPathForRow:0 inSection:(indexPath.row)]

   or

   let newIndexPath = NSIndexPath(forRow:0, inSection:indexPath.row)

   and uses this newIndexPath to obtain the object to display in the cell using the frc's objectAtIndexPath method.

Note that this remapping must be repeated/reversed in the other datasource methods. And it also means your tableView can have only one section.