How to Deal with Temporary NSManagedObject instances?

Question

I need to create NSManagedObject instances, do some stuff with them and then trash them or store to sqlite db. The problem is, I cannot create instances of NSManagedObject unconnected to NSManagedObjectContext and this means I have to clear up somehow after I decide that I don't need some of the objects in my db.

To deal with it, I have created an in-memory store using the same coordinator and I'm placing temporary objects there by using assignObject:toPersistentStore. Now, how do I ensure that these temporary objects don't get to the data, which I fetch from the common to both stores context? Or do I have to create separate contexts for such a task?

---

UPD:

Now I'm thinking about making separate context for in-memory store. How do I move objects from one context to another? Just using [context insertObject:]? Will it work OK in this setup? If I insert one object from the graph of objects, does the whole graph also get inserted into context?

Answer 1

NOTE: This answer is very old. See comments for full history. My recommendation has since changed and I no longer recommend using unassociated NSManagedObject instances. My current recommendation is to use temporary child NSManagedObjectContext instances.

Original Answer

The easiest way to do this is to create your NSManagedObject instances without an associated NSManagedObjectContext.

NSEntityDescription *entity = [NSEntityDescription entityForName:@"MyEntity" inManagedObjectContext:myMOC]; NSManagedObject *unassociatedObject = [[NSManagedObject alloc] initWithEntity:entity insertIntoManagedObjectContext:nil]; 

Then when you want to save it:

[myMOC insertObject:unassociatedObject]; NSError *error = nil; if (![myMoc save:&error]) {   //Respond to the error } 

Answer 2

iOS5 provides a simpler alternative to Mike Weller's answer. Instead use a child NSManagedObjectContext. It removes the need to trampoline through NSNotificationCenter

To create a child context:

NSManagedObjectContext *childContext = [[NSManagedObjectContext alloc] initWithConcurrencyType:NSMainQueueConcurrencyType]; childContext.parentContext = myMangedObjectContext; 

Then create your objects using the child context:

NSManagedObject *o = [NSEntityDescription insertNewObjectForEntityForName:@"MyObject" inManagedObjectContext:childContext]; 

The changes are only applied when the child context is saved. So to discard the changes just do not save.

There is still a limitation on relationships. ie You can't create relationships to objects in other contexts. To get around this use objectID's, to get the object from the child context. eg.

NSManagedObjectID *mid = [myManagedObject objectID]; MyManagedObject *mySafeManagedObject = [childContext objectWithID:mid]; object.relationship=mySafeManagedObject; 

Note, saving the child context applies the changes to the parent context. Saving the parent context persists the changes.

See wwdc 2012 session 214 for a full explanation.