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

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.