ios - Why doesn't NSManagedObject instances hold a strong reference to their NSManagedObjectContext?

Question

As pointed out in another question on SO (and the Apple docs), NSManagedObject instances do not hold a strong reference to the NSManagedObjectContext from which they originated. On first blush, this seems like a strange decision, since NSManagedObject instances are nearly useless without their context, since it leads to confusing bugs such as faults not firing when they should.

Can anyone provide some background on why this is the case? Would it be dangerous to implement an NSManagedObject subclass that automatically holds a strong reference to its NSManagedObjectContext?

Edit: thanks to great answers to this question, I've discovered that my managed objects were created against an intentionally temporary NSManagedObjectContext by RestKit. This follows into my next question, specific to RestKit, here.

Answer 1

It makes more sense for an NSManagedObjectContext to own its NSManagedObjects than the other way around.

Keep in mind that the context is like a draw pad with all its objects on it. If that context goes away, the objects are no longer valid. If the objects owned the context, then the context going away would do nothing to the objects and they still appeared to be valid. In other words: a context can exist without objects, objects can't exist without a context.

Of course, a hybrid model (where a context owns its objects and objects own their context) wouldn't work either, because then you would run into a retain cycle.

NSManagedObject instances are nearly useless without their context

They can be (though not necessarily), but remember that they do have a reference to their context! Presumably it's a weak reference, but a reference nonetheless. If that reference returns nil, the object is invalid. If you make sure your context stays around (which is what I did in my answer to the other question), you won't have any issues.