iPhone - Core Data Question?

I have 2 entities : User, Address. I have set the relationship each User can have many "Address". I am not sure how core data works but from my understanding, every time i call insertEntityForName it creates and stores an object. (also a row in table)

So the question: Doesn't the following code store a duplicate address in core data?

1. When i insert for user entity it also inserts an address
2. When i call insert for address entity it creates another address.

If i am correct and this actually creates a duplicate in the database what is the way to prevent it?

User *user = [NSEntityDescription insertEntityForName:@"User" 
                      inManagedObjectContext:self.managedObjectContext];
user.firstName = @"first name";
user.lastName = @"last name";

Address *address = [NSEntityDescription insertEntityForName:@"Address" 
                      inManagedObjectContext:self.managedObjectContext];
address.street = @"street";
user.address = address;

No, this will not create a duplicate address. With the first insert it only creates the User, not the Address. The user's address will be nil.

If you truly did make the Address relationship of the User one-to-many, you can't assign user.address like that, it should give a warning since it's expecting an NSSet*. Also I would recommend calling it addresses. You can do:

user.address = [NSSet setWithObject:address];

or

[user addAddressObject:address];