如何添加 "one-to-many" 的 "one-to" 部分以获取结果

Question

我希望能够将玩家的数据添加到许多关系的 "to-one" 部分。 fetch 为我做了一些聚合，但我想知道它属于什么player。

我有一个如下所示的 CoreData 模型：

我有一个如下所示的提取请求：

func statsPerPlayer(player: Players, managedContext: NSManagedObjectContext)  -> [String: Int] {

    var resultsDic = [String: Int]()

    let fetchRequest = NSFetchRequest<NSFetchRequestResult>(entityName: "Shifts")
    let predicate               = NSPredicate(format: "playersRelationship = %@", player)
    fetchRequest.predicate      = predicate

    let nsExpressionForKeyPath  = NSExpression(forKeyPath: "timeOnIce")

    let nsExpressionForFunctionMin = NSExpression(forFunction: "min:", arguments: [nsExpressionForKeyPath])
    let nsExpressionDescriptionMin = NSExpressionDescription()
    nsExpressionDescriptionMin.expression = nsExpressionForFunctionMin
    nsExpressionDescriptionMin.name = "minShift"
    nsExpressionDescriptionMin.expressionResultType = .integer16AttributeType

    let nsExpressionForFunctionMax = NSExpression(forFunction: "max:", arguments: [nsExpressionForKeyPath])
    let nsExpressionDescriptionMax = NSExpressionDescription()
    nsExpressionDescriptionMax.expression = nsExpressionForFunctionMax
    nsExpressionDescriptionMax.name = "maxShift"
    nsExpressionDescriptionMax.expressionResultType = .integer16AttributeType

    let nsExpressionForFunctionSum = NSExpression(forFunction: "sum:", arguments: [nsExpressionForKeyPath])
    let nsExpressionDescriptionSum = NSExpressionDescription()
    nsExpressionDescriptionSum.expression = nsExpressionForFunctionSum
    nsExpressionDescriptionSum.name = "sumShift"
    nsExpressionDescriptionSum.expressionResultType = .integer16AttributeType

    let nsExpressionForFunctionAvg = NSExpression(forFunction: "average:", arguments: [nsExpressionForKeyPath])
    let nsExpressionDescriptionAvg = NSExpressionDescription()
    nsExpressionDescriptionAvg.expression = nsExpressionForFunctionAvg
    nsExpressionDescriptionAvg.name = "avgShift"
    nsExpressionDescriptionAvg.expressionResultType = .integer16AttributeType

    let nsExpressionForFunctionCount = NSExpression(forFunction: "count:", arguments: [nsExpressionForKeyPath])
    let nsExpressionDescriptionCount = NSExpressionDescription()
    nsExpressionDescriptionCount.expression = nsExpressionForFunctionCount
    nsExpressionDescriptionCount.name = "countShift"
    nsExpressionDescriptionCount.expressionResultType = .integer16AttributeType

    fetchRequest.propertiesToFetch   = [nsExpressionDescriptionMin, nsExpressionDescriptionMax, nsExpressionDescriptionSum, nsExpressionDescriptionAvg, nsExpressionDescriptionCount]

    fetchRequest.resultType = .dictionaryResultType

    do {

        let fetchArray = try managedContext.fetch(fetchRequest)
        print(fetchArray)
        resultsDic = fetchArray.first as! [String : Int]

    } catch let error as NSError {

        print("\(self) -> \(#function): Could not fetch. \(error), \(error.userInfo)")
    }

    return resultsDic

}  //statsPerPlayer

结果看起来不错，像这样：

[{
    avgShift = 39;
    countShift = 4;
    maxShift = 89;
    minShift = 6;
    sumShift = 157;
}]

但是，我想包括此数据所针对的 player。如何在结果中添加 one-to-many 关系的 "to-one" 部分？

谢谢！！

Answer 1

在您上面的代码中，只需将相关关系名称添加到要获取的属性即可：

fetchRequest.propertiesToFetch   = ["playersRelationship", nsExpressionDescriptionMin, nsExpressionDescriptionMax, nsExpressionDescriptionSum, nsExpressionDescriptionAvg, nsExpressionDescriptionCount]

返回的字典将包含键 "playersRelationship"，其值设置为相应 Players 对象的 NSManagedObjectID。然后，您可以使用上下文的 object(with:) 方法访问 Players 对象本身。

更新

所以经过一些测试，结果是：

a) 如果在 propertiesToFetch 中包含关系，CoreData 会对 count 聚合函数感到困惑。这会导致 Invalid keypath (request for aggregate operation on a toOne-only keypath 错误。

b) 如果将关系包含在 propertiesToFetch 中，CoreData 会混淆所有其他聚合函数。（它计算每个对象的聚合，而不仅仅是那些匹配谓词的对象。）

这两个问题的解决方案是将关系添加为 GROUP BY 属性。然后 CoreData 正确地计算聚合并且也正确地将 count 识别为有效操作。因此，添加以下行：

fetchRequest.propertiesToGroupBy   = ["playersRelationship"]

如何添加 "one-to-many" 的 "one-to" 部分以获取结果

How to add "one-to" part of "one-to-many" to fetch results

core-data

one-to-many

nsfetchrequest

swift