NSManagedObject as NSDictionary key?

Question

In my app, I have a NSDictionary whose keys should be instances of a subclass of NSManagedObject.

The problem, however, is that NSManagedObject does not implement the NSCopying protocol which means that no Core Data objects / instances of NSManagedObject can be used as dictionary keys even though the -[hash] method works fine for them.

Was should I do?

Answer 1

Could you create a wrapper class, that contains a reference to the instance of NSManagedObject that you want to use as a dictionary key? You could then make this wrapper class implement NSCopying, along with a hash method (perhaps just calling the NSManagedObject's hash method), and use this wrapper as the dictionary key.

Answer 2

I suggest to use [[[myManagedObject objectID] URIRepresentation] absoluteString] as your key.

Answer 3

You have two options:

1. Stick the object ID into a dictionary instead and work with that. NSManagedObjectID is NSCopying-compliant so will properly handle this

2. NSDictionary's implementation boils down roughly to this internally:

   - (void)setObject:(id)anObject forKey:(id)aKey
   {
     id copiedKey = [aKey copy];
     CFDictionarySetValue(self, copiedKey, anObject);
     [copiedKey release];
   }

So you can just use CFDictionarySetValue() directly if you like.