title: Get sum of values from core data attributes displayed in an outline view populated from a tree controller
body: <p>I am attempting to learn more about creating apps for OS X and have run into an issue that google has not been able to provide an answer for.  The example app I am creating is an RSS reader.  I have Core Data Entities of Folder and Feed, with Feed inheriting from Folder.</p>
  
  <p>One attribute of Folder is unreadCount, with type Integer64.  Feed inherits this attribute from Folder.  The idea is that a display of the count of unread articles in a feed will be displayed and the total number of unread articles of all feeds contained in a folder will be displayed with that folder.</p>
  
  <p>I have an NSTreeController that is managing the Entity Folder.  This tree controller populates an outline view with the titles of the folders/feeds and the unreadCount for each feed and folder (right now folder is just defaulting to 0).  The unreadCount is getting displayed via a value transformer that takes the integer count and returns its string version.  All of this has bee set up with bindings and is working fine.</p>
  
  <p>The issue I am having is in getting the unreadCount of a folder to be the sum of all the unread counts in its feeds.  I have Googled for several hours and come across what appears to be a solution for tables populated from array controllers with the <code>@sum.value</code>, but this obviously does not work with tree controllers (from what I can determine anyway).</p>
  
  <p>So, my question, how do I get the displayed unreadCount for a Folder to be the sum of the unreadCounts of the Feeds the Folder contains when the display is an outline view populated from a tree controller?</p>