Question

我已经在谷歌和SE上找到了我无法找到问题的答案，所以如果我忽略了可能是一个简单的解决方案，我表示歉意。我还是绿色的，所以我可能没有使用正确的搜索字词。：）

好吧，我觉得我99％到了我想成为的地方。我有以下类函数：

class func fetchGroupTotal(_ group: Int, for managedObjectContext: NSManagedObjectContext) -> Double {
    let fetchRequest: NSFetchRequest<Group> = Group.fetchRequest()
    let keyPathExpression = NSExpression(forKeyPath: "accounts.balance")
    let sumExpression = NSExpression(forFunction: "sum:", arguments: [keyPathExpression])
    let expressionDescription = NSExpressionDescription()
    expressionDescription.name = "groupSum"
    expressionDescription.expression = sumExpression
    expressionDescription.expressionResultType = .decimalAttributeType
    fetchRequest.propertiesToFetch = [expressionDescription]
    fetchRequest.resultType = .dictionaryResultType

    do {
        let results = try managedObjectContext.execute(fetchRequest)
        print("\(results)")
        return results.value(forKey: "groupSum") as! Double
    } catch {
        fatalError("Error fetching SUM: \(error)")
    }
}

简而言之，我试图在sum关系上做accounts，而这部分似乎正在发挥作用。当我尝试访问我定义的groupSum属性时，会出现问题。我收到以下错误：

Terminating app due to uncaught exception 'NSUnknownKeyException', reason:
'[<NSAsynchronousFetchResult 0x600000288de0> valueForUndefinedKey:]: this class is not 
key value coding-compliant for the key groupSum.'

如何从结果中获取groupSum？

Answer 1

呼！那是一个艰难的时刻，但在谷歌搜索失败后，我终于找到了found my answer。一旦我对我fetchRequest的声明进行了更改，我就能够以groupSum的身份访问我的[NSDictionary]媒体资源。

对于那些可能感兴趣的人，这是我的函数的最终版本：

class func fetchTotal(_ group: Int, for managedObjectContext: NSManagedObjectContext) -> Double {

    // Right here is where the magic happens:
    let fetchRequest: NSFetchRequest<NSFetchRequestResult> = Group.fetchRequest()

    let keyPathExpression = NSExpression(forKeyPath: "accounts.balance")
    let sumExpression = NSExpression(forFunction: "sum:", arguments: [keyPathExpression])
    let expressionDescription = NSExpressionDescription()
    expressionDescription.name = "groupSum"
    expressionDescription.expression = sumExpression
    expressionDescription.expressionResultType = .decimalAttributeType
    fetchRequest.propertiesToFetch = [expressionDescription]
    fetchRequest.resultType = .dictionaryResultType
    fetchRequest.returnsDistinctResults = true

    do {
        let results = try managedObjectContext.fetch(fetchRequest).first
        if let result = results as? NSDictionary {
            return result.value(forKey: "groupSum") as! Double
        }
        return 0.00
    } catch {
        fatalError("Error fetching SUM: \(error)")
    }
}

KVC错误获取聚合属性

1 个答案: