Question

I have a question in core data:

there are 2 Entities in the project, Books and Pages;
there are 3 Objects books user created in Entity Books;
there are several Objects pages user created in Entity Pages;
relationship inbetween is one page belongs to one book, one book has many pages.

and my question: there are 3 book with the same object name “book”,and each has unique attribute .bookName : @"metal" ;@"plastic" ;@"glass". how to set page to the book with .bookName = @"glass" ?

    //In BookViewController
    Books *book = (Books *)[NSEntityDescription insertNewObjectForEntityForName:@"Books" inManagedObjectContext:managedObjectContext];
    //textView.text is user input text
    book.bookName = textView.text;


    //In PageViewController
    Pages *page = (Pages *)[NSEntityDescription insertNewObjectForEntityForName:@"Pages" inManagedObjectContext:managedObjectContext];
    page.itsbook = WHAT?

Thank you for reading, I stuck here like: a day, really appreciate your help, love you!

Answer 1

If you need to find a specific book then you need to use a NSFetchRequest to ask Core data for it.

Quite some code is needed, so you probably add a convinience method to your Bookclass that looks something like this:

+(Book*)bookWithName:(NSString*)name
{
    // 0. I assume you have something like this to get the context…
    NSManagedObjectContext* context = [NSManagedObjectContext threadLocalContext];

    // 1. Create an empty request
    NSFetchRequest* request = [[[NSFetchRequest alloc] init] autorelease];

    // 2. Set the entity description for the object to fetch
    NSEntityDescription* entity = [NSEntityDescription entityForName:@"Book"
                                              inManagedObjectContext:context];
    [request setEntity:entity];

    // 3. Set a predicate asking for objects with a mathing bookName property
    NSPredicate* predicate = [NSPredicate predicateWithFormat:@"%K LIKE[cd] %@", 
                                                              @"bookName",
                                                              name];
    [request setPredicate:predicate];

    // 4. Execute the request on the managed object context.
    NSArray* objects = [context executeFetchRequest:request error:NULL];

    // 5. Result is an array, maybe handle empty array and many objects?
    return [objects lastObject];
 }