CoreData sort on to-many relationship

Question

I'm writing an iOS app which has store of person records, and needs to display lists them sorted in particular ways. There are a variable number of these orderings, and they are generated on the fly, but I would like them to be stored in the datastore. The SQL way to do this is to have a ListPositions table with a list name, an id into the persons table, and a sort key. Then, to display a particular list, I can select all list ListPositions with a given name, pull in the referenced persons, and sort on the sort key. Trying to do this in CoreDatat, however I run into problems. I am trying to do this using a schema like:

Person:
    Name
    DOB
    etc... 
    positions -->> ListPosition

ListPosition:
    listName
    sortKey
    person --> Person

Then, I can get all the Persons in a given list with the NSPredicate [NSPredicate predicateWithFormat:@"ANY positions.listName like %@", someList]; This allows me to dynamically add lists against a large set of Persons. The problem is that I am unable to use the sortKey field of ListPosition to sort the Persons. What NSSortDescriptor will do this? And if it is not possible to sort a fetch on the property of one element of a to-many relationship, what is another way to get multiple, dynamic orderings in coredata? I am displaying the lists with a NSFetchedResultsController, so I can't put the lists together myself in memory. I need to do it with a single NSFetchRequest.

Answer 1

You're right-- following a to-many relationship returns an NSSet, which has no inherent sorting. To get sorted results there are a couple of options:

1. Assuming that Person/ListPosition is a two-way relationship, do a new fetch request for ListPosition entities. Make the predicate match on the "person" relationship from ListPosition, which would look something like [NSPredicate predicateWithFormat:@"person=%@", myPerson]. Use whatever sort descriptor you need on the fetch request.
2. Follow the relationship as you're doing, which gives you an NSSet. Then use NSSet's -sortedArrayUsingDescriptors: method to convert that to a sorted array.