在NSManagedObjectSubclass上通过Swift计算属性对NSFetchedResultsController进行排序 [英] Sorting NSFetchedResultsController by Swift Computed Property on NSManagedObjectSubclass

问题描述

我正在使用Swift和Core Data构建一个应用程序。在我的应用程序的某一点，我想有一个 UITableView 显示当前在持久存储中的类型的所有对象。目前，我正在检索它们，并使用 NSFetchedResultsController 在表视图中显示它们。我想通过我的 NSManagedObject 子类的计算属性对表视图进行排序，如下所示：

  class MHClub：NSManagedObject {
 @NSManaged var name：String 
 @NSManaged var shots：NSSet 
 var averageDistance：Int {
 get {
 if shots.count> 0 {
 var total = 0 
拍摄镜头{
 total + =（拍摄为！MHShot）.distance.integerValue 
} 
返回总计/拍摄。 count 
} 
 else {
 return 0 
} 
} 
} 
  

在我的表视图控制器中，我设置了 NSFetchedResultsController 的 fetchRequest 如下：

  let request = NSFetchRequest（entityName：MHClub）
 request.sortDescriptors = [ NSSortDescriptor（key：averageDistance，ascending：true），NSSortDescriptor（key：name，ascending：true）] 
  
 
 
这样设置会导致我的应用程序崩溃，在日志中出现以下消息：
 'NSInvalidArgumentException' ，reason：'keypath averageDistance not found in entity< NSSQLEntity MHClub id = 1>'
  
取出第一个排序描述符，我的应用程序运行正常，但我的表视图没有排序我想要它是如何。如何根据Swift中 NSManagedObject 子类的计算属性排序表视图？
解决方案
正如Martin所指出的，您不能对计算属性进行排序。只需更新俱乐部的每一次拍摄的新存储的属性。 
 
I'm building an app using Swift with Core Data. At one point in my app, I want to have a UITableView show all the objects of a type currently in the Persistent Store. Currently, I'm retrieving them and showing them in the table view using an NSFetchedResultsController. I want the table view to be sorted by a computed property of my NSManagedObject subclass, which looks like this:
class MHClub: NSManagedObject{
@NSManaged var name: String
@NSManaged var shots: NSSet
var averageDistance: Int{
    get{
        if shots.count > 0{
            var total = 0
            for shot in shots{
                total += (shot as! MHShot).distance.integerValue
            }
            return total / shots.count
        }
        else{
            return 0
        }
    }
}
In my table view controller, I am setting up my NSFetchedResultsController's fetchRequest as follows:
let request = NSFetchRequest(entityName: "MHClub")
request.sortDescriptors = [NSSortDescriptor(key: "averageDistance", ascending: true), NSSortDescriptor(key: "name", ascending: true)]
Setting it up like this causes my app to crash with the following message in the log:
'NSInvalidArgumentException', reason: 'keypath averageDistance not found in entity <NSSQLEntity MHClub id=1>'
When I take out the first sort descriptor, my app runs just fine, but my table view isn't sorted exactly how I want it to be. How can I sort my table view based on a computed property of an NSManagedObject subclass in Swift?
解决方案
As was pointed out by Martin, you cannot sort on a computed property. Just update a new stored property of the club every time a shot is taken. 

                        
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