CODEWITHSUNDEEP
ioscore-datanspredicate
Pascal
Pascal

Reputation: 2214

Core Data: filter many to may relationship with NSPredicate

I need to filter a many to many relationship. This means I have a third table for this connection.

Event <<- EventPlayerState ->> Player

I tried to find out if a Event already has the player connected via EventPlayerState.

The EventPlayerState table contains the attribute isActive to indicate if a player is active in the event.

I accessed the events relationship property hasEventPlayerStates. And tried to filter it with a NSPredicate. But it always returns false.

    NSPredicate *seachPlayerPredicate = [NSPredicate predicateWithFormat:@"(hasPlayer = %@)", [player objectID]];

    NSSet *playerEventStates = [event hasPlayerEventStates];

    NSSet *filteredPlayer = [playerEventStates filteredSetUsingPredicate:seachPlayerPredicate];

    if([filteredPlayer count] == 1) {
        return YES;
    }

    return NO;

What is wrong? Or is there a better way to do it?

Upvotes: 1

Views: 103

Answers (1)

Mundi
Mundi

Reputation: 80265

If you are not storing any additional information in your join table, that table to connect the two entities it is superfluous. You should then only have a many-to-many relationship between Player and Event.

The predicate is also much simpler: 

[NSPredicate predicateWithFormat:@"players.@count = 0"]

or to check for a particular Event

event.players.count == 0

In case you do need the join table, e.g because you are storing a timestamp or other information, it still looks sort of the same from the Event's point of view.

The corresponding checks are exactly equivalent: 

[NSPredicate predicateWithFormat:@"playerStates.@count = 0"]
event.playerStates.count == 0

Upvotes: 2

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