带核心数据的有序列表 [英] Ordered List With Core Data

问题描述

我正在使用同时是 NSManagedObject 和 Codable 的类。这是 Feed ，并且有多个相册。我想要相册的有序列表，但是Core Data迫使我改用Set，但没有排序。我可以使用 NSOrderedSet ，但是在 Codable 上不能很好地工作。在Core Data中获取有序列表的最佳方法是什么。下面是我要工作的模型。

I am working with a class which is both NSManagedObject and Codable. It's a Feed and it has multiple Albums. I want the ordered list of Albums but Core Data forces me to use Set instead, which is not ordered. I can use NSOrderedSet, but it doesn't work well with Codable. What would be the best way to get the ordered list in Core Data. Below is the Model I am trying to make work.

public class Feed: NSManagedObject, Codable {

    @nonobjc public class func fetchRequest() -> NSFetchRequest<Feed> {
        return NSFetchRequest<Feed>(entityName: "Feed")
    }

    @NSManaged public var title: String
    @NSManaged public var albums: Set<Album>
    @NSManaged public var feedContainer: FeedContainer?

    enum CodingKeys: String, CodingKey {
        case title
        case albums = "results"
        case feedContainer
    }

    public func encode(to encoder: Encoder) throws {
        var container = encoder.container(keyedBy: CodingKeys.self)
        try container.encode(title, forKey: .title)
        try container.encode(albums, forKey: .albums)
    }

    public required convenience init(from decoder: Decoder) throws {
        guard let contextUserInfoKey = CodingUserInfoKey.context,
            let managedObjectContext = decoder.userInfo[contextUserInfoKey] as? NSManagedObjectContext,
            let entity = NSEntityDescription.entity(forEntityName: "Feed", in: managedObjectContext) else {  fatalError("Failed to decode Subject!")  }
        self.init(entity: entity, insertInto: managedObjectContext)

        let container = try decoder.container(keyedBy: CodingKeys.self)
        title = try container.decodeIfPresent(String.self, forKey: .title) ?? ""
        albums = try container.decodeIfPresent(Set<Album>.self, forKey: .albums) ?? []
    }
}

推荐答案

我想要相册的有序列表

I want the ordered list of Albums

您的工作是 赋予相册实体一个要作为排序依据的属性。然后，在获取数据时，可以传递排序描述符以接收对该属性排序的相册。

It is your job to give the Album entity an attribute to sort by. Then, when you fetch, you can pass a sort descriptor to receive the Albums sorted on that attribute.

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