Entity relationship between NSManagedObjects issue - ios

Question

I'm running into a CoreData problem related to one-to-many relationships.
Entity 1 - Authors has a one-to-many relationship with Entity 2 - Books. I think Books has a one-to-one relationship with Authors.
Since there are multiple books per author in the author object I have
#property (nonatomic, retain) NSSet *book;
The corresponding property in the book object is:
#property (nonatomic, retain) Authors *author;
When the app downloads books from the server through an API, after saving the book, I am trying to also save the author and associate the book with the author with the following code:
NSEntityDescription *entity = [NSEntityDescription entityForName:#"Books" inManagedObjectContext:self.managedObjectContext];
NSManagedObject *record = [[NSManagedObject alloc] initWithEntity:entity insertIntoManagedObjectContext:self.managedObjectContext];
record setValue:bookname forKey:#"bookname"];
if ([self.managedObjectContext save:&error]) {
Authors *newAuthor = [NSEntityDescription insertNewObjectForEntityForName:#"Authors" inManagedObjectContext:self.managedObjectContext];
newAuthor.aid = authorid;
newAuthor.book = record;
}
This code has worked for me for one to one relationships, but in this case, is throwing following exception error:
'NSInvalidArgumentException', reason: 'Unacceptable type of value for
to-many relationship: property = "book"; desired type = NSSet; given
type = Books;
Can anyone suggest how to fix this?
Update:
I also tried switching it around to read:
record.author = newAuthor;
But this gives error
"property 'author' not found on object of type NSManagedObject"
although there is such a property defined in the Books object (as shown above).

Answer 1

Since the book property is a set, CoreData should have created an Authors method such as addBookObject:. When you created your custom entity classes, there should be a file with a name similar to "Authors+CoreDataProperties.h". Look in there for defined methods.
Your second option should also work if you use Books instead of NSManagedObject.
i.e.:
Books *record = [[NSManagedObject alloc] initWithEntity:entity insertIntoManagedObjectContext:self.managedObjectContext];

Answer 2

U can generate NSManagedObject subclass, after just create few objects, set relations and save context once, someting like:
NSEntityDescription *entity = [NSEntityDescription entityForName:#"Book" inManagedObjectContext:self.context];
Book *newBook = [[Book alloc] initWithEntity:entity insertIntoManagedObjectContext:self.context];
newBook.bookName = #"My new book";
NSEntityDescription *entity2 = [NSEntityDescription entityForName:#"Record" inManagedObjectContext:self.context];
Record *newRecord = [[Record alloc] initWithEntity:entity2 insertIntoManagedObjectContext:self.context];
newRecord.name = #"New record";
newBook.record = newRecord;
[newRecord addBook:[NSSet setWithObject:newBook]];
[self.context save:nil];
This is sample for db like below:
To autogenerate classes - select Entities in *.xcdatamodel file and press File->New->File select CoreData section and NSManaged Object subclass, go by wizard steps.
You will get something like
And even more:
Good tutorial u can also found here