Swift 3 Core Data - NSExpression forFunction：“sum：”抛出错误（“无法将字典转换为日”） [英] Swift 3 Core Data - NSExpression forFunction: "sum:" throws error ("could not cast dictionary to Day")

问题描述

我有天和任务。一天有很多任务。每个任务都有一个称为点的属性，我想对当天任务的所有点求和。我使用下面的代码（在Core Data中通过教程，swift 2版本找到），并试图修改它为swift 3（我也添加了一个谓词，但这不重要）。但是当我运行这个代码，我得到这个错误：

无法将值'NSKnownKeysDictionary1'（0x10d02d328）转换为'MyProject。日

我做错了什么？

 code> // sum current day's task points 
 let sumRequest：NSFetchRequest< Day> = Day.fetchRequest（）
 sumRequest.resultType = .dictionaryResultType 
 
 sumRequest.predicate = NSPredicate（format：SELF ==％@，argumentArray：[Project.Days.current]） 
 
 let expressionDescription = NSExpressionDescription（）
 expressionDescription.name =sumOfPoints
 expressionDescription.expression = NSExpression（forFunction：sum：，arguments：[NSExpression（forKeyPath： tasks.points）]）
 expressionDescription.expressionResultType = .integer16AttributeType 
 
 sumRequest.propertiesToFetch = [expressionDescription] 
 
 do {
 let results = try managedObject.fetch（sumRequest）as！ [NSDictionary] //这是导致错误的行
 print（DEN：，results）
} catch let错误为NSError {
 print（DEN：，error.localizedDescription ）
} 
  

我认为它与这一行有关：

  let sumRequest：NSFetchRequest< Day> = Day.fetchRequest（）
  

在这里，我明确地说结果将是一天new in swift 3？）。

解决方案

这解决了我的问题：

  let sumRequest：NSFetchRequest< NSFetchRequestResult> = Day.fetchRequest（）
  

而不是 Day 作为 NSFetchRequestResult ，我把 NSFetchRequestResult 本身。

我认为我将它设置为不同的 resultType 它不是 [Day] 不再返回，所以这就是为什么我需要更改为更通用或某事。

I have days and tasks. A day has many tasks. Every task has an attribute called "points" and I want to sum all points of the tasks of the current day. I used the code below (found in the book Core Data by tutorials, swift 2 version) and tried to modify it for swift 3 (I also added a predicate, but that's not important). But when I run this code, I get this error:

Could not cast value of type 'NSKnownKeysDictionary1' (0x10d02d328) to 'MyProject.Day'

What am I doing wrong?

    // sum current day's task points 
    let sumRequest: NSFetchRequest<Day> = Day.fetchRequest()
    sumRequest.resultType = .dictionaryResultType

    sumRequest.predicate = NSPredicate(format: "SELF == %@", argumentArray: [Project.Days.current])

    let expressionDescription = NSExpressionDescription()
    expressionDescription.name = "sumOfPoints"
    expressionDescription.expression = NSExpression(forFunction: "sum:", arguments: [NSExpression(forKeyPath: "tasks.points")])
    expressionDescription.expressionResultType = .integer16AttributeType

    sumRequest.propertiesToFetch = [expressionDescription]

    do {
        let results = try managedObject.fetch(sumRequest) as! [NSDictionary] // Here's the line that causes the error
        print("DEN:", results)
    } catch let error as NSError {
        print("DEN:", error.localizedDescription)
    }

I think it has something to do with this line:

let sumRequest: NSFetchRequest<Day> = Day.fetchRequest()

Here, I explicitly say the result will be a Day (I believe that's new in swift 3?). But I don't know how I could change this.

Thank you!

解决方案

This solves my problem:

let sumRequest: NSFetchRequest<NSFetchRequestResult> = Day.fetchRequest()

Instead of putting Dayas NSFetchRequestResult, I put NSFetchRequestResult itself there. Now it's working.

I believe as I set it to a different resultType it's not [Day]anymore that gets returned, so that's why I needed to change that to be more generic or something.

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