核心数据连接表，具有许多通过、获取的属性谓词

How do you write a Fetched Property for the Place Entity that will present an Array of Users?

对于获取的属性users of Place检索其签到与给定地点相关的所有用户，设置

• 所获取属性的目的地为“User”
• 以及“ANY checkins.event == $FETCH_SOURCE”的谓词

现在您可以获得某个地点的用户数组：

Place *place = ...;
NSArray *users = [place valueForKey:@"users"];

此获取的属性对应于以下获取请求：

Place *place = ...;
NSFetchRequest *request = [NSFetchRequest fetchRequestWithEntityName:@"User"];
NSPredicate *predicate = [NSPredicate predicateWithFormat:@"ANY checkins.event == %@", place];
[request setPredicate:predicate];
NSArray *users = [context executeFetchRequest:request error:&error];

如果您声明获取的属性users作为动态属性：

@interface Place (FetchedProperties)
@property(nonatomic, retain) NSArray *users;
@end

@implementation Place (FetchedProperties)
@dynamic users;
@end

然后您可以使用属性语法检索值：

NSArray *users = place.users;
// instead of: NSArray *users = [place valueForKey:@"users"];

But note您可以直接获得相同的结果（作为一组），而无需使用获取的属性：

Place *place = ...;
NSSet *users = [place.checkins valueForKey:@"user"];