From the docs:
You usually access to-many
relationships using
mutableSetValueForKey:, which returns
a proxy object that both mutates the
relationship and sends appropriate
key-value observing notifications for
you.
So this returns an “intelligent” NSMutableSet which automatically lets the context delete objects when they get deleted from the set, and reverse? Is that a proxy object?
Yes. See Accessor Search Pattern for Unordered Collections in the Key-Value programming guide for details.