如何通过关系进行核心数据查询?

假设演员和电影实体之间存在一对多的反向关系,您只需以与获取任何特定实体相同的方式获取Chuck Norris的实体,然后访问附加到演员实体上关系的电影实体数组。

// Obviously you should do proper error checking here... but for this example
// we'll assume that everything actually exists in the database and returns
// exactly what we expect.
NSEntityDescription *entity = [NSEntityDescription entityForName:@"Actor" inManagedObjectContext:self.managedObjectContext];
NSPredicate *predicate = [NSPredicate predicateWithFormat:@"name LIKE[c] 'Chuck Norris'"];
NSFetchRequest *request = [[NSFetchRequest alloc] init];
[request setEntity:entity];
[request setPredicate:predicate];

// You need to have imported the interface for your actor entity somewhere
// before here...
NSError *error = nil;
YourActorObject *chuck = (YourActorObject*) [[self.managedObjectContext executeFetchRequest:request error:&error] objectAtIndex:0];

// Now just get the set as defined on your actor entity...
NSSet *moviesWithChuck = chuck.movies;

值得注意的是,本例显然假设10.5使用属性,但您可以在10.4中使用访问器方法执行相同的操作。

或者您可以使用另一个谓词:

NSEntityDescription *entity = [NSEntityDescription entityForName:@"Actor" inManagedObjectContext:self.managedObjectContext];

NSPredicate *predicate = [NSPredicate predicateWithFormat:@"name = %@",@"Chuck Norris"]
NSFetchRequest *request = [[NSFetchRequest alloc] init];
[request setEntity:entity];
[request setPredicate:predicate];

YourActorObject *chuck = [[self.managedObjectContext executeFetchRequest:request error:nil] objectAtIndex:0];
[request release];

NSSet *moviesWithChuck = chuck.movies;