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?

score 73 · Accepted Answer · edited Dec 25 '13 at 13:36

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.

Thanks for this! I was working with Core Data in a background GCD thread, so had to have a background NSManagedObject, which I had forgotten to use to insert the parent object, so it was using the main thread NSManagedObject! Anyway, thanks! :) — runmad, Sep 25 '11 at 20:38

score 11 · Answer 2 · answered Feb 25 '13 at 18:46

I had the same problem and this sentence helped me to solve the error.

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.

Here's my code (I replaced the variables to make it work for you):

// Have the owner object and get the managedObjectContext
Owner *owner = [[DataFunctions alloc] getCurrentOwner];
NSManagedObjectContext *context = [owner managedObjectContext]; 

// Create a book and use the manageObjectContext of owner
Book *book = [NSEntityDescription insertNewObjectForEntityForName:@"Book" inManagedObjectContext:context];
[newQuote setValue: "Book Title" forKey:@"title"];
[newQuote setOwner:owner];

I hope this helps :)

score 4 · Answer 3 · answered Apr 18 '16 at 11:23

Here, you are trying to establish a relation between two objects which are fetched/created with a different context. Core data won't allow you to establish such relations. To achieve this, you should fetch the second object (you are trying to make a relationship with) with the context of the first object with objectID. Now you should be able to establish a relation between these two objects. For example:

MagicalRecord.saveWithBlock({[unowned self] (localContext: NSManagedObjectContext!) in

    let user = User.MR_createEntityInContext(localContext)!
    .....
    .....
}) //here local context stores data on end of block itself

MagicalRecord.saveWithBlock({[unowned self] (localContext: NSManagedObjectContext!) in

    let address = Address.MR_createEntityInContext(localContext)!
    .....
    .....
    let user = localContext.objectWithID(self.user!.objectID) as! User
    user.address = address
})

Hope this will help you!

score 2 · Answer 4 · answered Oct 12 '09 at 13:38

2

As the error says, you're not allowed to have a relationship in one Core Data object whose value is set to another object held in a different context. One way you can get around this is to wait until after you save the new object and merge it back into the primary context, then set the owner relationship as appropriate (since both objects are now in the same context, there's no problem with that).

answered Oct 12 '09 at 13:38

Tim

59,527
19
156
165

Following the Core Data Books example, the secondary MOC is released after the merge occurs. If I instead make the secondary MOC an instance variable, the primary MOC's owner is referring to books in the secondary MOC, which gives the error message. If I have a secondary MOC-owner that refers to books in the secondary MOC, when I merge I now have duplicate owner instances, which is not correct. Ideally I would be able to associate with an existing owner. If you have time, could you please explain your answer in a little more detail? – Alex Reynolds Oct 13 '09 at 11:05

score 1 · Answer 5 · answered Apr 18 '16 at 07:25

1

book *book = [mainContext ........]   //Get book from default context

NSManagedObjectID *objectId = [book objectID];

Book *tmpBook = [tmpContext objectWithID:objectId]; //Now book has the legal relationship

answered Apr 18 '16 at 07:25

Elangovan

1,158
1
9
26

score 1 · Answer 6 · answered Jun 01 '18 at 01:35

1

Swift version...

context.insert(objectFromOtherContext)

answered Jun 01 '18 at 01:35

Justin Domnitz

3,217
27
34

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

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