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.