Question

I have a problem with Core Data which has left me at the end of my tether. So I was wondering if any of you wonderful people could help.

Basically, I have two entities in a Core Data project. These are:

Author {authorName, dateOfBirth}
Book {bookName, pages}

There is a one-to-many relationship from author to books called ‘authors’, and an inverse relationship called ‘books’. I have exported the subclasses of these entities, and created my fetch controller. Do I now have to define the relationship programatically?

A list of authors is currently displayed in my table view. At the moment, I can only display a list of ALL the books in my Core Data project when I tap on an author. How would I go about accessing a list of books from a particular author? I am presuming I would use an NSPredicate, the logic I have for that so far is:

NSPredicate *predicate = [NSPredicate predicateWithFormat:@"ANY authors.bookName LIKE[cd] %@", authorName];

… but I am pretty sure that is incorrect.

I have been crawling the web for hours for an answer and remain confused. So any help would be greatly appreciated.

Many thanks 🙂

Answer 1

It’s likely that you have something like the following:

@interface AuthorListController : UITableViewController {
    UITableView *table;
    NSArray *authors; // array of NSManagedObjects
}

@property (nonatomic, retain) IBOutlet UITableView *table;
@property (nonatomic, retain) NSArray *authors;

And in your implementation you have:

-(void)tableView:(UITableView*)tableView didSelectRowAtIndexPath:(NSIndexPath*)indexPath {
    NSManagedObject *author = [authors objectAtIndex:indexPath.row];
    // create and show your book-level view here
}

Now that you’ve got the selected author object, you can get the books (assuming you’ve set up the one-to-many relationship in the Data Modeler) directly:

NSSet *books = author.books;

But if you didn’t have the author object loaded, for whatever reason, you might build a new search and use the books->authors relations in your predicate:

NSManagedObjectContext *objectContext = self.managedObjectContext;
NSEntityDescription *entity = [NSEntity entityForName:@"Books" inManagedObjectContext:objectContext];
NSFetchRequest *fetch = [[NSFetchRequest alloc] init];
[fetch setEntity:entity];

NSPredicate *predicate = [NSPredicate predicateWithFormat:@"author.name == %@", authorName];
[fetch setPredicate:predicate];

// ... create your error object and execute the fetch, etc