Core data join table, has many through, fetched properties predicate

Question

How do you write a Fetched Property for the Place Entity that will present an Array of Users?

Answer 1

For a fetched property users of Place that retrieves all users whose check-in is related to the given place, set

• the destination of the fetched property to "User"
• and the predicate to "ANY checkins.event == $FETCH_SOURCE"

Now you can get the array of users for a place:

Place *place = ...;
NSArray *users = [place valueForKey:@"users"];

This fetched property corresponds to the following fetch request:

Place *place = ...;
NSFetchRequest *request = [NSFetchRequest fetchRequestWithEntityName:@"User"];
NSPredicate *predicate = [NSPredicate predicateWithFormat:@"ANY checkins.event == %@", place];
[request setPredicate:predicate];
NSArray *users = [context executeFetchRequest:request error:&error];

If you declare the fetched property users as a dynamic property:

@interface Place (FetchedProperties)
@property(nonatomic, retain) NSArray *users;
@end

@implementation Place (FetchedProperties)
@dynamic users;
@end

then you can retrieve the value using property syntax:

NSArray *users = place.users;
// instead of: NSArray *users = [place valueForKey:@"users"];

But note that you can get the same result (as a set) directly, without using a fetched property:

Place *place = ...;
NSSet *users = [place.checkins valueForKey:@"user"];