UITableView populating Core data one-to-many relationship using checkmark - CoreData: error: Serious application error -

i have app i'm trying select resources (buildings, systems) programme relies on. relationship between entities resource.resourceusedonprog<<--->programme.usesresource. slice of data model here:

i'm trying use multiple checkmark selection on uitableviewcontroller of resources used particular programme. however, when try interact resource cells in table app crashes error

coredata: error: serious application error. exception caught during core data change processing. bug within observer of nsmanagedobjectcontextobjectsdidchangenotification. left hand side or operator must either nsarray or nsset. userinfo (null)

my code follows:

- (uitableviewcell *)tableview:(uitableview *)tableview cellforrowatindexpath:(nsindexpath *)indexpath {     static nsstring *cellidentifier = @"resource cell";      uitableviewcell *cell = [tableview dequeuereusablecellwithidentifier:cellidentifier];     if (cell == nil) {         cell = [[uitableviewcell alloc] initwithstyle:uitableviewcellstyledefault reuseidentifier:cellidentifier];     }      // configure cell...     resource *resource = [self.fetchedresultscontroller objectatindexpath:indexpath];     nsstring *fullname = [nsstring stringwithformat:@"%@ %@", resource.hasmanager.firstname, resource.hasmanager.surname];     cell.textlabel.text = resource.name;     cell.detailtextlabel.text = fullname;      if (resource.resourceusedonprog == selectedprogramme){ cell.accessorytype = uitableviewcellaccessorycheckmark;}     else {cell.accessorytype = uitableviewcellaccessorynone;}     return cell; }  - (void)tableview:(uitableview *)tableview didselectrowatindexpath:(nsindexpath *)path {     uitableviewcell *cell = [tableview cellforrowatindexpath:path];      resource *resource = [self.fetchedresultscontroller objectatindexpath:path];     if (cell.accessorytype == uitableviewcellaccessorycheckmark) {         cell.accessorytype = uitableviewcellaccessorynone;         resource.resourceusedonprog = null;     } else {         cell.accessorytype = uitableviewcellaccessorycheckmark;         resource.resourceusedonprog = selectedprogramme;          }    } 

the error seems triggered resource.resourceusedonprog = selectedprogramme; , resource.resourceusedonprog = null; events. if comment them out can check , uncheck cells (but clearly, not change state of resource.resourceusedonprog, i.e. i'm trying achieve).

i've found answer, based on 2 issues. firstly, relationship key nsset, not object, many-to-many relationship. secondly, nsmanagedobject subclass each entity automatically generates add , remove methods each relationship. in case of 1 i've implemented

[resource removeresourceusedonprogobject:selectedprogramme]; ,

[resource addresourceusedonprogobject:selectedprogramme]; 

the overall code here:

- (uitableviewcell *)tableview:(uitableview *)tableview cellforrowatindexpath:(nsindexpath *)indexpath {     static nsstring *cellidentifier = @"resource cell";      uitableviewcell *cell = [tableview dequeuereusablecellwithidentifier:cellidentifier];     if (cell == nil) {         cell = [[uitableviewcell alloc] initwithstyle:uitableviewcellstyledefault reuseidentifier:cellidentifier];     }      // configure cell...     resource *resource = [self.fetchedresultscontroller objectatindexpath:indexpath];     nsstring *fullname = [nsstring stringwithformat:@"%@ %@", resource.hasmanager.firstname, resource.hasmanager.surname];     cell.textlabel.text = resource.name;     cell.detailtextlabel.text = fullname;      if ([resource.resourceusedonprog containsobject:selectedprogramme]){ cell.accessorytype = uitableviewcellaccessorycheckmark;}     else {cell.accessorytype = uitableviewcellaccessorynone;}     return cell; }  - (void)tableview:(uitableview *)tableview didselectrowatindexpath:(nsindexpath *)path {     uitableviewcell *cell = [tableview cellforrowatindexpath:path];      resource *resource = [self.fetchedresultscontroller objectatindexpath:path];      nslog(@"selectedresource is: %@", resource.name);     if (cell.accessorytype == uitableviewcellaccessorycheckmark) {         cell.accessorytype = uitableviewcellaccessorynone;         [resource removeresourceusedonprogobject:selectedprogramme];         } else {         cell.accessorytype = uitableviewcellaccessorycheckmark;             [resource addresourceusedonprogobject:selectedprogramme];           }    }