Illegal attempt to establish a relationship 'xyz' between objects in different contexts

Question

I am using Apple's CoreDataBooks sample application as a basis for pulling data into a secondary managed object context in the background, and then merging that data into the primary managed object context.

The data I am pulling in is a Book entity with a to-one relationship with an Owner entity (called "owner"). The Owner entity has a to-many relationship with the Book (called "books").

My data is an XML document of the form:

<Owner>
  <Name>alexpreynolds</Name>
  <ID>123456</ID>
</Owner>
<Books>
  <Book>Book One</Book>
  <Book>Book Two</Book>
  ...   
  <Book>Book N</Book>
</Books>

Book One through Book N are associated with one Owner ("alexpreynolds, 123456").

I am parsing this into an Owner instance and an NSMutableSet made up of Book instances.

When I attempt to save the first time, it saves fine and the merged data shows up in the table view.

On the second save, however, when the XML content contains a new book, it doesn't work.

Here's what happens:

I then attempt to load in an XML document that contains a new Book not already in the primary managed object context. The new Book is using the same Owner as that which is associated with the other Books.

I have routines that pick out this unique Owner managed object (which I already have in my primary managed object context) and the unique Book that is not found in the primary MOC.

From this, I create a new Book object in the secondary MOC, and I set its "owner" relationship to point to the unique Owner I found in the primary MOC.

When I save, I get the following error:

*** Terminating app due to uncaught 
exception 'NSInvalidArgumentException', 
reason: 'Illegal attempt to establish a 
relationship 'owner' between objects in 
different contexts 

(source = <Book: 0x7803590> 
(entity: Book; id: 0x7802ae0 <x-coredata:///
Book/t527F06B2-3EB5-47CF-9A29-985B0D3758862>
; data: {
creationDate = 2009-10-12 06:01:53 -0700;
name = nil;
nameInitial = nil;
operations = (
);
owner = nil;
type = 0;
}) , 

destination = <Owner: 0x78020a0> (entity: 
Owner; id: 0x3a56f80 <x-coredata://043AF2F0-1AD0-
4078-A5E8-E9D7071D67D1/Owner/p1> ; data: {
books = "<relationship fault: 0x7801bf0 'books'>";
displayName = alexpreynolds;
ownerID = 123456;
}))'

How do I create a new Book entity in the secondary MOC, so that I can still associate it with a pre-existing Owner in the primary MOC?

Answer 1

You can't have relationships between objects in different managed object contexts. So one way of getting around that is to bring the object into the managed object context.

For example:

NSManagedObject *book = // get a book in one MOC
NSManagedObject *owner = // get an owner in a different MOC
[[owner mutableSetValueForKey:@"books"] addObject:[owner.managedObjectContext objectWithID:[book objectID]]];

So what you're doing is actually fetching the Book into the same managed object context with owner. Keep in mind, though, that this is only possible if book has already been saved. The managed object context is going to look for the object in the persistent store, so it has to be saved first.