Setting up a one to many relationship in Core Data on the iPhone.

Question

Hello all!

I am new to iPhone development trying to figure out how to setup a one to many relationship Core Data. I have two Entities setup One Leagues which has a one to many relationship to an entity Teams. So lot's of teams in a league. Think all the teams that play baseball and are in the MLB.

I am pre-filling the data when the user first enter's their username and password, so I have the leagues populate first, which I have no problem with. Then when I begin adding the teams I do a search looking for league. Code is below.

- (NSMutableSet *)checkItem:(int *)identifier inTable:(NSString *)table inContext:(NSString *)context {

    NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];

    NSEntityDescription *entity = [NSEntityDescription entityForName:table inManagedObjectContext:managedObjectContext];

    [fetchRequest setEntity:entity];

    NSPredicate *predicate = [NSPredicate predicateWithFormat:context,identifier];
    [fetchRequest setPredicate:predicate];

    NSError *error;
    NSArray *items = [managedObjectContext executeFetchRequest:fetchRequest error:&error];

    [fetchRequest release];

    if ([items count] >= 1) {
        return [items objectAtIndex:0];
    } else {
        return nil;
    }
}

By calling this and then set up the team entry and save ('league' is my relationship):

NSMutableSet *leagueObjectSet = [self checkItem:lid inTable:@"Leagues" inContext:@"id=%i"];

NSManagedObject *teamObject = [NSEntityDescription insertNewObjectForEntityForName:@"Teams" inManagedObjectContext:managedObjectContext];

[teamObject setValue:[NSNumber numberWithInt:tid] forKey:@"id"];
[teamObject setValue:[dict valueForKey:@"name"] forKey:@"teamName"];
[teamObject setValue:leagueObjectSet forKey:@"league"];

[self saveAction];

When I run this I get this error message in the console.

"the entity Teams is not key value coding-compliant for the key "league"."

Am I doing this right? With everything I have read it seems like I am. I come from a MySQL background so be gentle!

Answer 1

From your description, I believe that you have a wrong setup for your one to many relationship. Assuming you have two entities named League and Team, and that you have correctly setup the relationship, when you want to relate a team to a league, your league object must not be a NSMutableSet as shown in your code snippet: it must be a NSManagedObject too. A League object, being on the to many side of the relationship will have a property named teams or similar which will be a NSMutableSet. A Team object, on the one side of the relationship will have instead a property named league or similar which is a League object.

Then, to setup the relationship you can do as you did using key value coding, but for performance reasons it is best to do

teamObject.id = yourId;
teamObject.teamName = @"your team name";
teamObject.league = yourLeagueObject;

This is much faster and suggested by Apple.