Question

I have three entities: Session, User and Test. A session has 0-many users and a user can perform 0-6 tests. (I say 0 but in the real application always at least 1 is required, at least 1 user for a session and at least 1 test for a user. But I say 0 to express an empty start.) All entities have their own specific data attributes too. A user has a name, A session has a name, a test has six values to be filled in by the user, and so on. But my issue is with the relationships.

1. How do I set multiple users and have them added to one session (same goes for multiple tests for one user).

2. How do I show the content in a right way? How do I show a session that has multiple users and these users having completed multiple tests?

Here’s my code so far with regard to issue 1:

Session *session = [NSEntityDescription insertNewObjectForEntityForName:@"Session"
                                  inManagedObjectContext:context];
session.name = @"Session 1";

User *users = [NSEntityDescription insertNewObjectForEntityForName:@"User"
                                   inManagedObjectContext:context];
users.age = [NSNumber numberWithInt:28];
users.session = session;
//session.user = users;
[sessie addUserObject:users];

With regard to issue 2: I can log the session, but I can’t get the user(s) logged from a session.

NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
NSEntityDescription *entity = [NSEntityDescription entityForName:@"Session"
                                          inManagedObjectContext:context];
[fetchRequest setEntity:entity];
NSArray *fetchedObjects = [context executeFetchRequest:fetchRequest error:&error];
for (Session *info in fetchedObjects) {
 NSLog(@"Name of session: %@", info.name);
 NSLog(@"Having problems with this: %@",info.user);
 //User *details = info.user;
 //NSLog(@"User: %@", details.age);
}

Answer 1

I find it useful (and it seems to be common practice from the code samples that I saw at SO), to use the plural form for to-many relationships, e.g. users for the to-many relationship from Session to User. It emphasizes the fact that the value of the relationship is a set and not a single object, and might make things a bit clearer.

So your model would look like this:

Issue 1: If you have created a Session *session and a User *user, then

user.session = session;

adds the user to the session. Calling

[session addUsersObject:user];

has the same effect. But only one of these calls is required, it automatically implies the other, if the relationships are properly defined as inverse relationships.

Issue 2: For a Session *session, session.users is the set of all users related to that session. It is a NSSet and you can iterate through that set. Similarly, user.tests is the set of all tests for a user.

So the following code displays all sessions with their users and tests:

NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
NSEntityDescription *entity = [NSEntityDescription entityForName:@"Session"
                                          inManagedObjectContext:context];
[fetchRequest setEntity:entity];

NSArray *sessions = [context executeFetchRequest:fetchRequest error:&error];
for (Session *session in sessions) {
    NSLog(@"Name of session: %@", session.name);
    for (User *user in session.users) {
        NSLog(@"   User name %@, age %@", user.name, user.age);
        for (Test *test in user.tests) {
            NSLog(@"      Test: %@", test.name);

        }
    }
}