ios - setPropertiesToFetch returns 'Invalid keypath expression' error -


i trying use group-by core data functionality in basic form:

- (nsfetchedresultscontroller *)fetchedresultscontroller {      nsmanagedobjectcontext *managedobjectcontext = [self managedobjectcontext];      if (_fetchedresultscontroller != nil) {         return _fetchedresultscontroller;     }      nsexpression *reviewnumkeyexpression = [nsexpression expressionforkeypath:@"review_num"];     nsexpression *reviewmaxexpression = [nsexpression expressionforfunction:@"max:" arguments:[nsarray arraywithobject:reviewnumkeyexpression]];      nsexpressiondescription *maxreviewed = [[nsexpressiondescription alloc] init];     [maxreviewed setname:@"maxreview"];     [maxreviewed setexpression:reviewmaxexpression];     [maxreviewed setexpressionresulttype:nsinteger32attributetype];      nsfetchrequest *fetchrequest = [[nsfetchrequest alloc] init];     nsentitydescription *entity = [nsentitydescription                                    entityforname:@"group" inmanagedobjectcontext:managedobjectcontext];      [fetchrequest setresulttype:nsdictionaryresulttype];     [fetchrequest setreturnsdistinctresults:yes];     [fetchrequest setentity:entity];      nsattributedescription *groupid = [entity.propertiesbyname objectforkey:@"group_id"];     nsarray *propertiestofetch = [nsarray arraywithobjects:groupid, maxreviewed, nil];     [fetchrequest setpropertiestofetch:propertiestofetch];     [fetchrequest setpropertiestogroupby:propertiestofetch]; ...

when running code, following error:

terminating app due uncaught exception 'nsinvalidargumentexception', reason: 'invalid keypath expression ((), name maxreview, isoptional 1, istransient 0, entity (null), renamingidentifier maxdepartment, validation predicates ( ), warnings ( ), versionhashmodifier (null) userinfo { }) passed setpropertiestofetch

i guess missing api, appreciated!

i think error message wrong: problem in setpropertiestogroupby, not setpropertiestofetch. cannot group computed value (maxreview), doubt want to: if want maximum value of review_num each groupid, need groupid in propertiestogroupby:

nsattributedescription *groupid = [entity.propertiesbyname objectforkey:@"group_id"]; nsarray *propertiestofetch = [nsarray arraywithobjects:groupid, maxreviewed, nil]; nsarray *propertiestogroupby = [nsarray arraywithobjects:groupid, nil]; [fetchrequest setpropertiestofetch:propertiestofetch]; [fetchrequest setpropertiestogroupby:propertiestogroupby];

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