ios - 尝试向 Core Data 添加新记录时出错

Question

我想将数据保存到 CoreData，但出现下一个错误:

Terminating app due to uncaught exception 'NSUnknownKeyException',
reason: '[<NSEntityDescription 0x7fc428f75940> setValue:forUndefinedKey:]: 
this class is not key value coding-compliant for the key username.'

这是我的代码:

let appDelegate: AppDelegate = UIApplication.sharedApplication().delegate as! AppDelegate
    let context: NSManagedObjectContext = appDelegate.managedObjectContext

    let request: NSFetchRequest = NSFetchRequest(entityName: "UnreadMessages")
    request.returnsObjectsAsFaults = false

    do {
        let result: NSArray = try context.executeFetchRequest(request)
        let newRecord: NSEntityDescription = NSEntityDescription.entityForName("UnreadMessages", inManagedObjectContext: context)!

        if result.count > 0 {
            print("It exists")
            print(context)
        } else {
            print("it's a new record")
            newRecord.setValue(user.jidStr, forKey: "username")
            newRecord.setValue("1", forKey: "count")

            do {
                try context.save()
            } catch let error {
                print(error)
            }
            print(context)
        }
    } catch _ {

    }

直到:

print("it's a new record")
newRecord.setValue(user.jidStr, forKey: "username")

打印 It's a new record 并崩溃!我的代码有什么问题？

这是我的 CoreData 截图:

Answer 1

Terminating app due to uncaught exception 'NSUnknownKeyException', reason: '[ setValue:forUndefinedKey:]: this class is not key value coding-compliant for the key username.'

这是您的重要线索。这意味着您要求 NSEntityDescription 的实例使用 KVC 设置名称为“username”的属性，但该类不支持该键。

prints It's a new record and crashes! What is wrong with my code?

好吧，您的代码不会创建新的“记录”。这段代码

let newRecord: NSEntityDescription = NSEntityDescription.entityForName("UnreadMessages", inManagedObjectContext: context)!

创建一个 NSEntityDescription，它可以用来创建一个 NSManagedObject。

创建一个你想调用的新“记录”

let newRecord = NSEntityDescription.insertNewObjectForEntityForName(
                    "UnreadMessages",
                    inManagedObjectContext:context)

因此，您现在可以看到您正在尝试在实体描述而非托管对象上设置“用户名”。