Error saving to many relationship to core data - core-data

Question

I am trying to get my head around the core data and I am trying to implement a small to-many relationship but I keep getting the error below:
I have two NSManagedObject classes (Groups and Contacts) created by Xcode's data model with a to-many relationship.
#implementation Groups
#dynamic groupId;
#dynamic groupName;
#dynamic groupContacts;
#end
#implementation Contacts
#dynamic firstName;
#dynamic lastName;
#dynamic userId;
#dynamic belongsToGroup;
I have contacts data as an array of dictionaries and I am doing the following:
self.localGroup = [NSEntityDescription insertNewObjectForEntityForName:#"Groups" inManagedObjectContext:self.scratchPadContext];
self.localGroup.groupName = #"Some Name";
for (int i = 0; i < [self.ContactsData count]; i++) {
Contacts *contact = [NSEntityDescription insertNewObjectForEntityForName:#"Contacts" inManagedObjectContext:self.scratchPadContext];
NSDictionary *obj = (NSDictionary *)[self.ContactsData objectAtIndex:i];
contact.firstName = [obj valueForKey:#"firstName"];
contact.userId = [obj valueForKey:#"email"];
[self.localGroup addGroupContactsObject:contact];
}
if (![self.scratchPadContext save:&error]) {
NSLog(#"Errror saving Group ********************************* %#, %#", error, [error userInfo]);
}
I get the following Error:
[4459:15503] Errror saving Group *****************
Error Domain=NSCocoaErrorDomain Code=134030 "The operation couldn’t be
completed. (Cocoa error 134030.)" UserInfo=0x85e0ab0
{NSAffectedObjectsErrorKey=(
" (entity: Groups; id: 0x838d020 ; data:
{\n groupContacts = nil;\n groupId = 0;\n groupName = Some
Name;\n})" ), NSUnderlyingException=Cannot update object that was
never inserted.}, {
NSAffectedObjectsErrorKey = (
" (entity: Groups; id: 0x838d020 ; data:
{\n groupContacts = nil;\n groupId = 0;\n groupName = Some
Name;\n})"
);
NSUnderlyingException = "Cannot update object that was never inserted."; }
If I comment out the relationship part and just save the group name, it works fine. So, I guess the problem is with the relationship data but I cannot figure out what. Can anyone please point me to where I could be doing it wrong?
[EDIT]:
When I add
contact.belongsToGroup = self.localGroup;
I get the following error even though I am using one MOC. I cannot understand where the mistake is. I would greatly appreciate if anyone can shed some light on this.
*** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: 'Illegal attempt to establish a relationship 'belongsToGroup' between objects in different contexts (source = <Contacts: 0x86926a0> (entity: Contacts; id: 0x8692700 <x-coredata:///Contacts/t2548922E-A0C6-4E3C-9760-3265954764E73> ;

Answer 1

Have you tried saving the scratchPadContext before adding the contacts to the group (that is, creating the localGroup object, saving the context, then adding the contacts, and saving the context again? From the error message, it looks like Core Data is sending an update statement to the permanent store before it sends the necessary insert, so it may be getting tripped up because you're writing both ends of a new to-many relationship to the permanent store at the same time.