Core Data Relationships: How to insert a new object into an entity and create a relationship to an existing object in another entity

Question

I am building a little iPhone app which allows users to keep score of games they may be playing with friends. I now need to use relationships in Core Data but can't quite seem to get it working.

I want to be able to add new data into one entity while creating a relationship to existing data in a different entity. How can I achieve this?

Please note I am new to Core Data and have spent the best part of today trying to figure this out but have run out of luck. Any help would be very much appreciated.

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I have 3 entities: Scores, Games and Players.

Scores Attributes: date, player1Score, player2Score and status.

Games Attributes: title.

Players Attributes: name.

I have many to many relationships between (Scores <<--->> Games) and (Scores <<--->> Players)

---

I already have a list of both games and players. The user selects which game and who is playing and with this information a set of objects is created in the Scores entity with relationships to the chosen game and players.

Here is my source:

//  Scores.h

#import <Foundation/Foundation.h>
#import <CoreData/CoreData.h>

@class Games, Players;

@interface Scores : NSManagedObject

@property (nonatomic, retain) NSDate * date;
@property (nonatomic, retain) NSNumber * player1Score;
@property (nonatomic, retain) NSNumber * player2Score;
@property (nonatomic, retain) NSNumber * status;
@property (nonatomic, retain) NSSet *game;
@property (nonatomic, retain) NSSet *player;
@end

@interface Scores (CoreDataGeneratedAccessors)

- (void)addGameObject:(Games *)value;
- (void)removeGameObject:(Games *)value;
- (void)addGame:(NSSet *)values;
- (void)removeGame:(NSSet *)values;

- (void)addPlayerObject:(Players *)value;
- (void)removePlayerObject:(Players *)value;
- (void)addPlayer:(NSSet *)values;
- (void)removePlayer:(NSSet *)values;

@end

---

// SC_ScoreViewController.h

#import <UIKit/UIKit.h>

@interface SC_ScoreViewController : UIViewController

@property (strong) NSIndexPath *game;
@property (strong) NSIndexPath *playerOne;
@property (strong) NSIndexPath *playerTwo;

@property (weak, nonatomic) IBOutlet UILabel *playerOneName;
@property (weak, nonatomic) IBOutlet UILabel *playerTwoName;

@end

---

//  SC_ScoreViewController.m

#import "SC_ScoreViewController.h"
#import "Scores.h"
#import "Players.h"
#import "Games.h"

@interface SC_ScoreViewController ()

@end

@implementation SC_ScoreViewController

@synthesize game;
@synthesize playerOne;
@synthesize playerTwo;

// managedObjectContext (context)
- (NSManagedObjectContext *)managedObjectContext {
    NSManagedObjectContext *context = nil;
    id delegate = [[UIApplication sharedApplication] delegate];
if ([delegate respondsToSelector:@selector(managedObjectContext)]) {
        context = [delegate managedObjectContext];
    }
    return context;
}

- (id)initWithNibName:(NSString *)nibNameOrNil bundle:(NSBundle *)nibBundleOrNil
{
    self = [super initWithNibName:nibNameOrNil bundle:nibBundleOrNil];
    if (self) {
        // Custom initialization
    }
    return self;
}

- (void)viewDidLoad
{
    [super viewDidLoad];

    // Setup Nav Title
    self.navigationItem.title = [self.game valueForKey:@"title"];

    // Setup Player's Names
    [self.playerOneName setText:[self.playerOne valueForKey:@"name"]];
    [self.playerTwoName setText:[self.playerTwo valueForKey:@"name"]];


    Scores * newEntry = [NSEntityDescription insertNewObjectForEntityForName:@"Scores"        inManagedObjectContext:self.managedObjectContext];
    newEntry.player1Score = 0;
    newEntry.player2Score = 0;
    newEntry.status = nil;
    newEntry.player = self.playerOne; // Incompatible pointer types assigning to NSSet from NSIndexPath

    NSError *error;
    if (![self.managedObjectContext save:&error]) {
        NSLog(@"Whoops, couldn't save: %@", [error localizedDescription]);
    }

    - (void)didReceiveMemoryWarning
    {
    [super didReceiveMemoryWarning];
    // Dispose of any resources that can be recreated.
    }

    @end

---

I hope I have been clear and provided enough information. This is my first question so I hope all is in order. Any help would be amazing, thank you.

Answer 1

You have to insert Players' objects into self.managedObjectContext exactly the same as you insert a new Score object. It could be something like this:

Scores *score = [NSEntityDescription insertNewObjectForEntityForName:@"Scores" inManagedObjectContext:self.managedObjectContext];
Player *player = [NSEntityDescription insertNewObjectForEntityForName:@"Player" inManagedObjectContext:self.managedObjectContext];
[score addPlayerObject:player];

You also have to change your relationship "Scores <<--->> Players" to be ordered, because right now you won't know which player is which. Other option here would be to use two many-to-one relationship (separately for player1 and player2).

One more thing: it's a best practice to name your entities in a singular form, so Score, instead of Scores and so on.

You should read Creating Managed Object Relationships. It's all nicely described in there.

UPDATE: I think there shouldn't be a many-to-many relationship between Games and Scores (from what I understand Score can be renamed to Match). It probably should be a one-to-many relationship.