Question

所以我正在阅读Apple的核心数据指南和Stack上的问题，这一切都是正确的，但它并没有完全“点击”我的脑袋。

我有一个BookShop，它有一对多的书籍关系。

我进入了一家名为“Jenny's BookShop”的BookShop和一家名为“BigWorld Books”的书店。

这就是我将“新书”添加到“新书店”的方式：

BookShop *myBookShop = [NSEntityDescription insertNewObjectForEntityForName:@"BookShop" inManagedObjectContext:context];

myBookShop.books = [[NSMutableSet alloc]init];

Book *myBook = [NSEntityDescription insertNewObjectForEntityForName:@"Book" inManagedObjectContext:context];

myBook.BookShop = myBookShop;

[myBookShop addBookObject：myBook];

说我想要一本书只属于“BigWorld Books”而不是“Jenny's BookShop”。

我假设我使用获取请求并获取指向“BigWorld Books”的指针，然后将书分配给这个特定的书店？这就是我不断搞砸的地方。

如果有人能够“慢慢地”解释这一点，那将非常感激。

修改

答案：

        NSPredicate *predicate = [NSPredicate predicateWithFormat:@"name == %@"];
        [request setPredicate:predicate];
        [request setFetchLimit:1];
        BookShop *resultBookShop = [[_managedObjectContext executeFetchRequest:request error:&error] lastObject];
        [myBook setBookShop:resultBookShop];

Answer 1

//if BookShop with name BigWorld Books not existed, create. If existed, use it
BookShop *bookshop = [self isExistedEntityWitName:@"BigWorld Books" entity:@"BookShop"];
if(!bookshop){
  //insertNewObjectForEntityForName
}else{
  //bookshop is existed
  //insert new book
  Book *b = [NSEntityDescription insertNewObjectForEntityForName:@"Book"
                                             inManagedObjectContext:context];
  //add new book (b) to NSSet of bookshop, and you have NSSet *books
  bookshop.books = books;
}

ios核心数据获取特定值

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