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Core Data One-Way Relationships

Tom Ryan wrote this on March 06, 2013 under coredata, dev, ios, mobile-development .

Core Data Makes Things Simple

I really love using Core Data. It's made writing a data store-backed app pretty dead easy.

There are times when it can cumbersome, and I'd like to take a look at one such example.

Core Data relationships need to be two-way. This can be annoying when you want to store an object as a property, and seemingly the only way to do that is to create a model for that object and define a relationship for it.

But you don't always want a back-relationship.

Modeling One-Way Relationships

Let's say I want to model a User.

This User will have a first name, last name, and a bunch of attributes that may differ from user to user -- for example, an NSDictionary with possibly different sets of keys.

First and last names are easy enough to handle -- just make their property type NSStrings. The heterogeneous attributes are a different matter. Core Data doesn't allow objects like dictionaries as raw properties, so we have to do some magic with NSValueTransformer.

So, once the NSManagedObject subclass has been generated, it's often easiest to set the property type to the actual type you'll be storing. In this case, it's an NSDictionary.

@interface User : NSManagedObject

    @property (nonatomic, retain) NSString * firstName;
    @property (nonatomic, retain) NSString * lastName;
    //@property (nonatomic, retain) id payload;
    /* Changing the property type to NSDictionary so I don't have to worry 
       about casting it when I call it */
    @property (nonatomic, retain) NSDictionary *payload;


Note: I generally like to make any changes to the NSManagedObject class in a category, so when I have to make changes to the model, I won't have to worry about overwriting any changes I've made.

Now it's time to create the PayloadTransformer class, which is a subclass of NSValueTransformer:

#import "PayloadTransformer.h"

    @implementation PayloadTransformer
    + (BOOL)allowsReverseTransformation {
        return YES;
    + (Class)transformedValueClass {
        return [NSData class];
    - (id)transformedValue:(id)value {
        NSData *data = [NSKeyedArchiver archivedDataWithRootObject:value];
        return data;
    - (id)reverseTransformedValue:(id)value {
        NSDictionary *payload = [NSKeyedUnarchiver unarchiveObjectWithData:value];
        return payload;

We're Done!

Excellent, we're done! So, how is this actually used?

- (void)addUser {
      User *user = [NSEntityDescription insertNewObjectForEntityForName:@"User" inManagedObjectContext:self.managedObjectContext];
      user.firstName = @"Wil";
      user.lastName = @"Wheaton";
      NSDictionary *userPayload = @{@"Favorite Color":@"Blue", @"Best Buddy":@"Floxotron"};
      user.payload = userPayload;
      NSError *saveError = nil;
      if(![self.managedObjectContext save:&saveError]) {
        NSLog(@"Hey, error! %@", saveError);

    - (void)getWil {
      NSFetchRequest *wilRequest = [[NSFetchRequest alloc] init];
      wilRequest.entity = [NSEntityDescription entityForName:@"User" inManagedObjectContext:self.managedObjectContext];
      wilRequest.predicate = [NSPredicate predicateWithFormat:@"firstName=%@ AND lastName=%@", @"Wil", @"Wheaton"];
      NSError *wilError = nil;
      User *wilWheaton = [[self.managedObjectContext executeFetchRequest:wilRequest error:&wilError] lastObject];
      if(wilWheaton) {
        NSDictionary *payload = wilWheaton.payload;
        NSLog(@"Payload: %@", payload);

This is the simplest way I've found to simulate a one-way relationship in Core Data. Enjoy!