How to delete a temporary object on a child managed object context?

Question

I have a CodeData model [Phone]<<--->>[Forwarding]. So the Phone object has a Forwardings set, and vice versa.

I have a list of Phones and want to add one of them to a new Forwarding.

In the ForwardingViewController I do:

// Create a new managed object context; set its parent to the fetched results controller's context.
managedObjectContext = [[NSManagedObjectContext alloc] initWithConcurrencyType:NSMainQueueConcurrencyType];
[managedObjectContext setParentContext:[fetchedResultsController managedObjectContext]];
self.forwarding = (ForwardingData*)[NSEntityDescription insertNewObjectForEntityForName:@"Forwarding"

So this creates a child MOC and a now temporary Forwarding.

Then I pass self->forwarding to my PhonesViewController which shows all Phones (in a table). This view controller is simply navigation-pushed.

When the user taps on one of the Phones in the table I do:

[self.forwarding addPhonesObject:phone];

The addPhonesObject is a CoreData generated accessor.

Now, when the user is back at the ForwardingViewController and taps the Cancel button (because he decides he does not want to create a new Forwarding after all), it is dismissed, which cleans up this child managedObjectContext and also self.forwarding.

After doing the above, I get a database error (Cocoa error 1550). When trying to understand the console output, my guess is that the Forwarding was indeed deleted, but that the Phone object (which of course is still there), now has a null reference to this deleted Forwarding.

My question. How should I handle this case correctly: Having a temporary object created on a child MOC, link it to another object (on the parent MOC), and then delete this temporary object again.

Marcus S. Zarra Marcus S. Zarra · Accepted Answer · 2014-02-08T19:40:38

What is the actual error you are getting?

From your description, I am guessing that your PhonesViewController is listing phones from a different NSManagedObjectContext than the one that you created the ForwardingData entity from. This violates the relationship rule with Core Data. The rule is simple, to create a relationship between two entities they must both be from the same NSManagedObjectContext instance.

I question why you are creating a temporary NSManagedObjectContext in this situation. Since you are retaining the ForwardingData entity and you know when you are being cancelled, it seems cleaner to just delete the temporary entity when cancel is pressed instead of standing up another NSManagedObjectContext.

Update

If you need to use the child (per your comment), then you should change your PhonesViewController to accept a NSManagedObjectContext via dependency injection. Then you can send it the same NSManagedObjectContext instance as the one you used to create the new entity. With that change everything will work as you expect it to.

How to delete a temporary object on a child managed object context?

1 Answers

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