objective-c - 核心数据 : keypath name not found in entity

Question

这条消息让我崩溃了:

'NSInvalidArgumentException', reason: 'keypath name not found in entity

显然我没有正确查询我的实体。

//fetching Data

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

NSManagedObjectContext *context = [(AppDelegate *)[[UIApplication sharedApplication] delegate] managedObjectContext];

NSEntityDescription *entity = [NSEntityDescription entityForName:@"Viewer" inManagedObjectContext:context];
[fetchRequest setEntity:entity];

NSString *attributeName = @"dF";

NSPredicate *predicate = [NSPredicate predicateWithFormat:@"name like %@",attributeName];
[fetchRequest setPredicate:predicate];

NSLog(@"predicate : %@",predicate);
NSError *error;
NSArray *items = [context executeFetchRequest:fetchRequest error:&error];
NSLog(@"items : %@",items);

[fetchRequest release];
    
//end of fetch

这是我的数据模型:

我想返回“dF”的值，不应该这样调用吗？ :

NSString *attributeName = @"dF";
NSPredicate *predicate = [NSPredicate predicateWithFormat:@"name like %@",attributeName];

Answer 1

如果你想从你的 dF 属性中获取值，你必须获取一个 NSManagedObjects 数组，然后使用 [fetchedManagedObject valueForKey:@"dF"]; 来获得你的值(value)。

NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
NSManagedObjectContext *context = [(AppDelegate *)[[UIApplication sharedApplication] delegate] managedObjectContext];
NSEntityDescription *entity = [NSEntityDescription entityForName:@"Viewer" inManagedObjectContext:context];
[fetchRequest setEntity:entity];
NSArray *items = [context executeFetchRequest:fetchRequest error:&error];
[fetchRequest release];

NSManagedObject *mo = [items objectAtIndex:0];  // assuming that array is not empty
id value = [mo valueForKey:@"dF"];

谓词用于获取满足您的条件的 NSManagedObjects 数组。例如。如果您的 dF 是一个数字，您可以创建像“dF > 100”这样的谓词，然后您的提取请求将返回一个包含 NSManagedObjects 的数组将具有大于 100 的 dF 值。但是如果您只想获取值，则不需要任何谓词。