如何使用Codable处理Coredata中的重复条目

Question

此init函数每次都插入一个新实体，即使user_id相同。

我该如何解决？我希望每个实体仅与user_id成为一个。

我的代码是这样的：

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

    let userInfo = try decoder.container(keyedBy: UserInfoKeys.self)
    username = try userInfo.decodeIfPresent(String.self, forKey: .username)
    user_id = try userInfo.decode(String.self, forKey: .user_id)
}