I found this question on the CoreDataQuestions page, and it was right up front lacking an answer. I also have the exact same question, and was hoping someone could shed light on the issue:
When you dial into an array through a key path, we have operators to do aggregation, for example like company.projects.@count or produc ts.@avg.prices . When I try to use these on the MutableSets returned by CoreData relations, it can’t give me a count, and the log reports that the @count key path is not supported. What key path can I use if I want to know the number of objects on the other side of a to-many relation? Or am I completely missing something? It seems that a FetchedProperty might do this, but XCode rejects all of the predicates I try to build to do this.
*I got this to work by subclassing the “owner” of the to-many relation thus: *
+(void)initialize { [MQUsage setKeys:[NSArray arrayWithObject:@”cues”] triggerChangeNotificationsForDependentKey:@”cueCount”]; /* “cues” is the key to my related set */
[super initialize];
}
-(unsigned int)cueCount { return [[self valueForKey:@”cues”] count]; }
It seems like there should be a simple binding key for getting the cardinality of a to-many without having to write mehtods, but “_NSFaultingMutableSet” doesn’t appear to be KVO compliant for -count.