objective-c - 核心数据 : only the last object can be initialized correctly in a loop

Question

我的模型有两个实体 Artist 和 Album，Album 有一个 Artist 实例成员。我使用以下代码预填充我的模型，但只发现最后一张专辑，即 ablum3，已与 Artist Beatles 建立了正确的关联。对于album1、album2，artist字段为nil。

一定是哪里出了问题，我没发现...

//create  an artist
NSManagedObject *artist = [NSEntityDescription
                           insertNewObjectForEntityForName:@"Artist"
                           inManagedObjectContext:__managedObjectContext]; 

[artist setValue:@"Beatles" forKey:@"name"];

//populate the data
NSArray *albums = [NSArray arrayWithObjects:@"album1",@"album2",@"album3", nil];
for (NSString *title in albums){
    //populate the data
    NSManagedObject *album = [NSEntityDescription
                              insertNewObjectForEntityForName:@"Album"
                              inManagedObjectContext:__managedObjectContext]; 

    [album setValue:title forKey:@"title"];
    [album setValue:artist forKey:@"artist"];
}

Answer 1

没有进一步的细节，很难知道发生了什么。我试着理解你所写内容的模型。

所以，这个模型适合我

albums 是与 Album 的一对多关系。此外，它是可选的，您可以有 Artist 而没有 Album。

artist 是艺术家的反向关系。一对一的基数。这是必需的，因为如果没有 Artist，您将无法拥有 Album。

这里是代码:

- (void)populateDB
{
    //create  an artist
    NSManagedObject *artist = [NSEntityDescription
                               insertNewObjectForEntityForName:@"Artist"
                               inManagedObjectContext:[self managedObjectContext]]; 

    [artist setValue:@"Beatles" forKey:@"name"];

    //populate the data
    NSArray *albums = [NSArray arrayWithObjects:@"album1",@"album2",@"album3", nil];
    for (NSString *title in albums){
        //populate the data
        NSManagedObject *album = [NSEntityDescription
                                  insertNewObjectForEntityForName:@"Album"
                                  inManagedObjectContext:[self managedObjectContext]]; 

        [album setValue:title forKey:@"title"];
        [album setValue:artist forKey:@"artist"];
    }
}

调用populatedDB后，调用[self saveContext]保存上下文

- (void)saveContext {
    NSError *error = nil;
    NSManagedObjectContext *moc = [self managedObjectContext];
    if (moc != nil) {
        if ([moc hasChanges] && ![moc save:&error]) {
             // Replace this implementation with code to handle the error appropriately.
             // abort() causes the application to generate a crash log and terminate. You should not use this function in a shipping application, although it may be useful during development. 
            NSLog(@"Unresolved error %@, %@", error, [error userInfo]);
            abort();
        } 
    }
}

如果您需要安排您的模型，请告诉我。

希望对您有所帮助。