IOS Swift Core Data 如何在 propertiesToFetch 中添加不在 propertiesToGroupBy 中的字段

Question

需要帮助。

我的表中有 4 个字段:

• 电子邮件
• 留言
• 阅读
• date_received

我要选择email , message (recent) , date_received, and unread messages

这是我的预期结果:

测试@email.com |测试留言 | 2015-02-27 | 28test2@email.com |测试消息2 | 2015-02-29 | 2

这是我当前的代码:

let fetchRequest:NSFetchRequest = NSFetchRequest()

if let entityDescription:NSEntityDescription = NSEntityDescription.entityForName("Message", inManagedObjectContext: managedObjectContext){
    fetchRequest.entity = entityDescription
}

fetchRequest.propertiesToFetch = ["email","message","read","date_received"]
fetchRequest.propertiesToGroupBy = ["email"]
fetchRequest.resultType = .DictionaryResultType
fetchRequest.returnsObjectsAsFaults = false

let items:NSArray = managedObjectContext .executeFetchRequest(fetchRequest, error: nil)!

输出:

20 18:24:51.639 JPtxt[33368:1345477] *** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: 'SELECT clauses in queries with GROUP BY components can only contain properties named in the GROUP BY

Answer 1

只需使用通常的 NSManagedObjectResultType 获取消息(您无需指定)。然后通过 KVC 获取计数(两种解决方案都经过测试):

let count = (result.filteredArrayUsingPredicate(
   NSPredicate(format: "read = NO")) as NSArray).count

一种非标准但可能更简洁的方法是利用 bool 值存储为 1 和 0 的事实

let count = result.count - result.valueForKeyPath("@sum.read")!.integerValue