Question: I have User and Friend entities in my data model, being a one User to many Friends relationship.

My ViewController, is instantiated with an instance variable for User (\*user), and therefore I can access all friends by loading user.friends as friends is defined as an NSSet in my User object.

In my code I load all friends in an NSMutableArray, do some stuff, and potentially before leaving want to add additional friends, and edit the attributes of existing friends. I'm at a loss on how I can add/edit friends.

Should I edit the NSMutableArray of friend objects, and save that back to the User.context? If so, how? 

If editing a friend, should I copy existing friend object, change values, delete old object from the array and add the new (copied and updated) one?

I hope this makes sense...

Answer: 
You can modify your Friend objects (no need to make new copies and delete old ones).

Try this:

```
// create a mutable copy of an array from the set of the user's friends
NSMutableArray *friends = [[user.friends allObjects] mutableCopy];

// modify friends array or any Friend objects in the array as desired

// create a new set from the array and store it as the user's new friends
user.friends = [NSSet setWithArray:friends];
[friends release];

// save any changes
NSManagedObjectContext *moc = [user managedObjectContext];
if ([moc hasChanges] && ![moc save:&error]) {
    // handle error
}

```

You could also use a mutable set instead of an array: 

```
// create a mutable copy of the set of the user's friends
NSMutableSet *friends = [user.friends mutableCopy];

// modify friends set or any Friend objects in the set as desired

// create a new set from the set and store it as the user's new friends
user.friends = [NSSet setWithSet:friends];
[friends release];

// save any changes
NSManagedObjectContext *moc = [user managedObjectContext];
if ([moc hasChanges] && ![moc save:&error]) {
    // handle error
}

```
If you have a pointer/reference to a NSManagedObject you can edit all attributes. Say you get one friend like this:

```
Friend *aFriend = [user.friends anyObject];
[aFriend setLastName:@"new Lastname"];
NSError *error = nil;
[[self managedObjectContext] save:&error];

```

To add a friend do this:

```
Friend *aNewFriend = [NSEntityDescription insertNewObjectForEntityForName:@"friend" inManagedObjectContext:[self managedObjectContext]];
[user addFriendsObject:aNewFriend];
NSError *error = nil;
[[self managedObjectContext] save:&error];

```

However this will not add the new friend to the NSMutableArray that you created from the user.friends set earlier.