What dies it mean that a fetched property is static and not dynamically updating itself?

Question

Apple says in the NSFetchedPropertyDescription class reference:

Note, however, that unlike a playlist a fetched property is static—it does not dynamically update itself as objects in the destination entity change.

Nice, but what exactly does that mean: static property? Not dynamically updating in the destination entity change?

Can someone explain that?

What I get in my head is that a fetched property is for weak references. Like: A playlist may reference songs, but the playlist doesnt "own" them. If the playlist gets deleted, the song's never get deleted as a consequence of that. Also, the songs have no back-reference to the playlist, they don't have to know that it even exists.

Ok, so if a song gets deleted, the playlist isn't notified in any way? Or what is Apple trying to tell me?

Answer 1

Essentially it means that you have to manually update the fetched result every time to receive the latest changes of your source object.

I just consulted your quoted documentation. The next paragraph after your quote should make it clear:

The effect of a fetched property is similar to executing a fetch request yourself and placing the results in a transient attribute, although with the framework managing the details. In particular, a fetched property is not fetched until it is requested, and the results are then cached until the object is turned into a fault. You use refreshObject:mergeChanges: (NSManagedObjectContext) to manually refresh the properties—this causes the fetch request associated with this property to be executed again when the object fault is next fired.