Creating a memory-only NSManagedObject model - ios

Question

I need to create an instance of a NSManagedObject that will not be saved in CoreData and only in memory.
e.g.:
I have the Item and Log NSManagedObject, and they have relations between them.
I want to be able to create a Log instance without any core data properties, and assign its item property to an Item instance.
I know I can create it in a different, memory-persistence, context (or nil context). But then I can't assign the item property, since my Item instance is in the core-data context.
NSEntityDescription *description = [NSEntityDescription entityForName:#"Log" inManagedObjectContext:defaultContext];
Log *log = [[Log alloc] initWithEntity:description insertIntoManagedObjectContext:nil];
log.item = item;
This code throws an exception when ran:
Illegal attempt to establish a relationship 'item' between objects in different contexts
How can I achieve this in another way?

Answer 1

You can create a NSManagedObjectContext with parent context set to your Log's MOC.

Answer 2

Do you need the relationship to have an inverse? If not, you could use the ObjectID for the item objects as an attribute in the Log entity. You would need to convert the ObjectID to its URIRepresentation, and then convert that to a NSString:
NSURL *itemURI = [item.objectID URIRepresentation];
NSString *itemURIstring = [itemURI absoluteString];
log.itemURI = itemURIstring;
(If item has not yet been saved to the database, it will have a temporary ID - you should test for this with item.objectID.isTemporary otherwise the objectID may change). When you want to find the Item object for a given Log object, reverse the process:
NSURL *itemURI = [NSURL URLwithString:log.itemURI];
NSManagedObjectID *itemObjectID = [self.context.persistentStoreCoordinator managedObjectIDForURIRepresentation:itemURI];
Item *item = [self.context objectWithID:itemObjectID];
Pretty cumbersome!
I guess if you need an inverse, you could do the same (i.e. store a URI for the Log object as a string in Item).