我正在对createdAt系统字段进行排序，如下所示：
query.sortDescriptors = [NSSortDescriptor(key: "createdAt", ascending: false)]
并得到以下错误：
CKError 0x1c4447fb0：“无效参数”（12/2018）；服务器消息 = "未知字段'createdAt'"; uuid = 9C450848-2449-4892-93BC-C46363203042；容器 ID = "...
我能够查询和排序我以这种方式创建的字段。系统领域有什么不同吗？该字段的索引是可排序和可查询的。
改为使用creationDate钥匙。CK 仪表板中的元键与用于查询的元键略有不同。以下是元键列表：
recordID: CKRecordID
The unique ID of the record.
recordType: String
The app-defined string that identifies the type of the record.
creationDate: Date?
The time when the record was first saved to the server.
creatorUserRecordID: CKRecordID?
The ID of the user who created the record.
modificationDate: Date?
The time when the record was last saved to the server.
lastModifiedUserRecordID: CKRecordID?
The ID of the user who last modified the record.
recordChangeTag: String?
A string containing the server change token for the record.