So, in an app we have two NSManagedObjectContext's, lets call them context1 and context2. We have a situation in which an object, with customId=1, is inserted into context2, and context2 is never saved. At some point in the future an object is added to context1, with customId=1 also. context1 is then saved and when the completion notification is received the fun begins! We try to merge the changes from the save into context2 via:
[context2 mergeChangesFromContextDidSaveNotification:notification];
This works fine, it does the merge and then there are two objects in context2 both with customId=1. However, what I want to happen is, on merge, it somehow realises that both of the objects have the same customId and so instead of doing an insert, it just updates the existing object and internally makes the two the same object (or something to that effect :/). I had thought this may be possible by overriding isEqual and hash, but this is strictly forbidden for NSManagedObjects!
Another thought was to use validateInsert: and when it tries to insert the new object tell it not to and copy over the values. This however, causes another problem. We now have a persistent store with one object and context2 has a different object. We would then have to delete the object from context1 and save that change to remove the object from the persistent store... But since we never want to save context2 (this may seem odd, but we have valid reasons... I promise !) that object would then never be saved.
We basically want to be able to tell CoreData that after two inserts have been made they are actually supposed to be the same object! If anyone has ideas on how we may be able to do this, any help at this point would be greatly appreciated!
That type of merge strategy is something you need to deal with and is outside of the scope of the framework. Basically you have a dirty sandbox and a clean sandbox. When a change is made in the clean sandbox it will get propagated to the dirty one.
It is the responsibility of the owner of the dirty sandbox to watch for changes coming in and react to them. You can listen for the NSManagedObjectContextDidSaveNotification and check for a collision. From there it is your business logic that determines what happens next.