ios - FetchedResultsController NSSortDescriptor for one-to-many relationship -


i have 2 entities: chat<->> chatmessages (each chat has 0+ messages, each message belongs 1 chat).

i'm making fetchedresultscontroller , i'd sort chats date of last message:

    nsentitydescription *entity = [nsentitydescription entityforname:@"chatmodel" inmanagedobjectcontext:[nsmanagedobjectcontext mr_defaultcontext]];     [fetchrequest setentity:entity];      nssortdescriptor *sort = [[nssortdescriptor alloc] initwithkey:@"chatmessages.messagedate" ascending:no];     [fetchrequest setsortdescriptors:[nsarray arraywithobject:sort]];

but code get: 

*** terminating app due uncaught exception 'nsinvalidargumentexception', reason: 'to-many key not allowed here'

how can sort chats in required way? (just whatsapp)

i would need @ least 2 things.

use nspredicate fetch specific chat:

nspredicate *predicate = [nspredicate predicatewithformat:@"chatidentifier == %@", achatidentifier]; [fetchrequest setpredicate:predicate];

and change nssortdescriptor this:

nssortdescriptor *sort = [[nssortdescriptor alloc] initwithkey:@"messagedate" ascending:no];

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