核心数据 - 相关记录的数量

Question

我是Core Data编程的新手,我正在尝试解决所涉及的概念.我有一个名为a的对象Lease,它与Apartment对象有多对多的关系.给定一个Lease对象,如何获取相关Apartment对象的计数或相关对象的NSArray？

谢谢!

~~加思

Answer 1

有几种不同的方式:

1. 使用生成的Core Data访问器:

   NSSet * apartments = [myLease apartments];
   NSUInteger numberOfApartments = [apartments count];

2. 使用KeyPaths:

   NSSet * apartments = [myLease valueForKey:@"apartments"];
   NSUInteger numberOfApartments = [apartments valueForKey:@"@count"];

3. 使用KVC(如果您的课程完全符合KVC标准):

   NSUInteger numberOfApartments = [myLease countOfApartments];

4. 使用获取请求:

   NSFetchRequest * r = [[NSFetchRequest alloc] init];
   [r setEntity:apartmentEntityDescription];
   [r setPredicate:[NSPredicate predicateWithFormat:@"lease = %@", myLease]];
   NSArray * apartments = [myManagedObjectContext executeFetchRequest:r error:nil];
   NSUInteger numberOfApartments = [myManagedObjectContext countForFetchRequest:r error:nil];
   [r release];

随意混合和匹配这些线.