Core Data: Query objectIDs in a predicate?

Question

Hi there,

I am fetching a set of objects from a Core Data persistent store using a fetch request and a predicate. My current predicate simply checks whether an attribute is >= a certain value. This all works great, except that I want to finally exclude any objects that are currently held in an array.

I basically need to be able to exclude a set of objects, and the only way I think I can do this is to be able to get a list of objectID's from my managed objects array, and create another expression in my predicate to ensure that any objects returned don't have the same objectID. I.E. "ANY records.objectID NOT IN %@", arrayOfObjectIDs

How can I do this?

Any ideas?

Thanks,

Mike

Answer 1

A predicate like

NSPredicate *predicate = [NSPredicate predicateWithFormat:@"NOT (self IN %@)", arrayOfExcludedObjects];

where the entity of the fetch request is the entity of objects in the array, should do what you want. This can, of course be combined with other clauses in a single predicate for a fetch request.

In general, object comparisons (e.g self == %@ or self IN %@) compare on objectID in Core Data queries. The argument can either be an NSManagedObject instance or an NSMangedObjectID instance. So the predicate format above could take arrayOfExcludedObjects or [arrayOfExcludedObjects valueForKey:@"objectID"] as the argument.