Use NSManagedObject in child NSManagedObjectContext instead of its parent

Question

I have two MOCs: the first is the root context. When I save this context, the changes are saved to the persistent store coordinator. The second MOC has the first MOC as the parent. When I save the second MOC, I also have to save the first MOC in order to save the changes in the second MOC to the persistent store coordinator.

I use the second MOC to let the user edit an object. He can save or cancel the changes. When he saves the changes, all MOCs are saved. When he cancels the changes, I call rollback() of the second MOC.

Unfortunately, the object comes from the first MOC. This means, I execute an NSFetchRequest to fetch the object on the first MOC. Then I create the second MOC in which the user can edit the object. But there is a problem: when the second MOC should change something, for example delete an object that is contained in an array of the original object the user wants to edit, this is not possible, because a MOC can only delete objects that have this MOC as the context. But the object was fetched in the first MOC.

That's why I need to "transfer" somehow the object from the first MOC to the second MOC before the user edits the object. I don't want to fetch the object again with a NSFetchRequest or something, there must be a better way…

Is this possible? Or do you recommend to do this completely different, maybe without parent contexts?

Answer 1

This is where the objectID property of NSManagedObject will come in handy.

• Ask the object for its ID

  let objectID = myManagedObject.objectID
  

• Ask the child context for a managed object with that ID

  do {
      let childManagedObject = try childContext.existingObjectWithID(objectID)
      print("\(newObject)")
  } catch {
  
  }
  

Answer 2

I think you might be complicating your time with this. Unless it is completely necessary for proposed changes to also be saved there should be no reason to have two contexts for this. There are multiple ways for you to handle a temporary data which you can use to compare your actual record to without having it stored twice.

Why don't you just create a copy of the NSManagedObject and handle the information correction by comparison or simply replacing the original NSManagedObject with the data of the copy and then save it? I personally like this setup a bit more since all I have to do is either compare individual properties when update is desired.

When a full update is required than you can simply work directly on the one NSManagedObject without worrying about copies since you will probably be replacing the entire thing anyway. Like I said, there are other ways to handle, but if it is absolutely necessary for you to have both contexts then looking for a comparison of each property value to the replaced value and then simply save the one in the parent context.