神奇的记录 - 在循环中向实体添加值

Question

我的 Magical Record 代码有问题：

    Collection *collection = [Collection MR_createEntity];
    [collection setValue:name forKey:@"name"];
    [collection setValue:date forKey:@"date"];
    [collection setValue:amount forKey:@"amount"];

    for (Person *person in self.selectedPeople) {
        Payment *payment = [Payment MR_createEntity];
        [payment setValue:person forKey:@"person"];
        [payment setValue:collection forKey:@"collection"];
    }

   [[NSManagedObjectContext MR_defaultContext]  MR_saveToPersistentStoreWithCompletion:^(BOOL success, NSError *error) {
        if (!success)
            NSLog(@"Error: %@", [error localizedDescription]);
        else
            NSLog(@"Item %@ added to database", collection.name);
    }];

对于用户选择的每个人，我想在数据库中创建付款。我正在循环中这样做。我的问题是只有最后一次付款有收款。

例如，当我有 3 个人时，它看起来像这样：

Payment 1 - person:John, collection: nil
Payment 2 - person:Bill, collection: nil
Payment 3 - person:Mark, collection: Paper

谁能告诉我为什么会这样？

Answer 1

我的猜测是，这是从收款到付款的关系。您正在尝试使用从收款到付款的多对一，但您的核心数据模型中可能只有一对一设置。

我不确定从收款到付款的多对一关系是否是个好主意。您可能希望为每个集合创建一个唯一实体，但这完全取决于您的模型。

for (Person *person in self.selectedPeople) {
    // Create a new collection entity for each payment.
    Collection *collection = [Collection MR_createEntity];
    [collection setValue:name forKey:@"name"];
    [collection setValue:date forKey:@"date"];
    [collection setValue:amount forKey:@"amount"];

    Payment *payment = [Payment MR_createEntity];
    [payment setValue:person forKey:@"person"];
    [payment setValue:collection forKey:@"collection"];
}

[[NSManagedObjectContext MR_defaultContext]  MR_saveToPersistentStoreWithCompletion:^(BOOL success, NSError *error) {
    if (!success)
        NSLog(@"Error: %@", [error localizedDescription]);
    else
        NSLog(@"Item %@ added to database", collection.name);
}];