objective c - NSPredicate to fetch last message for each conversation in core data -

i have entity named messages following attributes: -> message_id, message_from, message_to, message_body, date_created.

i want fetch last message each user have in whatsapp chat.

i found following code mysql, don't know how use nspredicates:

select * messages m1 date_created = (select max(m2.date_created)                       messages m2                       m1.message_from = m2.message_from                         , m1.message_to = m2.message_to) 

could please me last message each user core data using nspredicate?

first of need method extracts messages , users. simplicity put in appdelegate.m. note getallmessages gets unique results extracting data nsdictionary definition not allow duplicate keys

-(nsarray*)getallusers {     // initializing nsfetchrequest     nsfetchrequest *fetchrequest = [[nsfetchrequest alloc] init];      //setting entity queried     nsentitydescription *entity = [nsentitydescription entityforname:@"message"                                           inmanagedobjectcontext:self.managedobjectcontext];     fetchrequest.resulttype = nsdictionaryresulttype;     fetchrequest.returnsdistinctresults = yes;     [fetchrequest setentity:entity];     [fetchrequest setpropertiestofetch:@[@"userid"]];     fetchrequest.sortdescriptors = @[[nssortdescriptor sortdescriptorwithkey:@"date_created" ascending:no]];      nserror* error;      // query on managedobjectcontext generated fetchrequest     nsarray *fetchedrecords = [self.managedobjectcontext executefetchrequest:fetchrequest error:&error];      // returning fetched records     return fetchedrecords; }  -(nsarray*)getallmessages {     // initializing nsfetchrequest     nsfetchrequest *fetchrequest = [[nsfetchrequest alloc] init];      //setting entity queried     nsentitydescription *entity = [nsentitydescription entityforname:@"message"                                           inmanagedobjectcontext:self.managedobjectcontext];     fetchrequest.sortdescriptors = @[[nssortdescriptor sortdescriptorwithkey:@"date_created" ascending:no]];     [fetchrequest setentity:entity];     nserror* error;      // query on managedobjectcontext generated fetchrequest     nsarray *fetchedrecords = [self.managedobjectcontext executefetchrequest:fetchrequest error:&error];      // returning fetched records     return fetchedrecords; } 

then can extract , put last message each user in nsmutablearray doing this:

appdelegate * appdelegate = (appdelegate *)[[uiapplication sharedapplication] delegate]; nsmutablearray * arrayusers = [nsmutablearray arraywitharray:[appdelegate getallusers]]; nsmutablearray * arraymessages = [nsmutablearray arraywitharray:[appdelegate getallmessages]];  nsmutablearray * lastmessageforeachuser = [nsmutablearray array];//on array results  (nsdictionary * dict in arrayusers) {     nsnumber * userid = [dict objectforkey:@"userid"];     nspredicate * pr = [nspredicate predicatewithformat:@"userid == %@",userid];     nsarray * results = [arraymessages filteredarrayusingpredicate:pr];     message * lastmessage = results.count>0 ? [results objectatindex:0] : nil;     [lastmessageforeachuser addobject:lastmessage];  }