What's the difference between -existingObjectWithID:error: and –objectWithID: ?

Question

What's the difference between -existingObjectWithID:error: and –objectWithID: ?

Answer 1

I believe existingObjectWithID:error: returns an instance only if it has already been registered with the receiving context. objectWithID: may return a fault for an object that has not yet been registered (pulled into) the receiving context.

Answer 2

objectWithID: assumes the object exists so if you give it a bad ID it will throw an exception when you attempt to access a property on the returned entity. Also, this method always returns an entity.

existingObjectWithID:error: will return an object if it exists and nil if it does not. If there was an error it will populate the error pointer.