使用属性搜索实体时的核心数据SubEntitiesByName

Question

我试图从核心数据中获取一个实体,具体取决于名为"name"的属性.这是我的代码:

NSManagedObjectContext *context = [self managedObjectContext];



    NSEntityDescription *vegetable = [NSEntityDescription insertNewObjectForEntityForName:@"Vegetable" inManagedObjectContext:context];

    // get the vegetable using name

    NSFetchRequest *request = [[NSFetchRequest alloc] init];

    NSPredicate *predicate =
    [NSPredicate predicateWithFormat:@"name == %@", myGardenViewModel.vegetable.name];

    [request setEntity:vegetable];
    [request setPredicate:predicate];

    NSError *error; 

    NSArray *fetchedObjects = [self.managedObjectContext executeFetchRequest:request error:&error];

最后一行抛出以下异常:

Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '-[Vegetable subentitiesByName]: unrecognized selector sent to instance 0x6bbcdd0'

有任何想法吗？

Answer 1

问题是你设置你的entity属性的方式NSFetchRequest.随着insertNewObjectForEntity:你实际上是在创建一个新的对象被插入到数据库后用[managedObjectContext save:&error];.

你想要做的是只为你的获取请求设置实体,而不是插入一个新对象:

NSEntityDescription *vegetable = [NSEntityDescription entityForName:@"Location" 
   inManagedObjectContext:self.managedObjectContext];