Calculated value not being updated correctly using donejs

I have standard donejs project with a component and a model both created using the default generators like this:

donejs add component storyBoard story-board
donejs add supermodel story

I left a sample project here: https://github.com/riescorp/donejs-promise-iterate

If you run this example using donejs develop and go to http://localhost:8080 you'll see something like this:

The problem is that whenever you change a value of the number of pages (input boxes) for any story, the sum won't update its value. I tried almost everything, from promises to already-resolved-promises but nothing seems to do the trick.

Somehow it seems that when any of those values change, there is no update trigger at all. I also tried setting the results as a List (can.List) but with no luck.

The problem seems to be that elem.pages should be elem.attr("pages").

Here's a version of the code that works for me:

export const ViewModel = Map.extend({
  define: {
    storyPromise: {
      get: function() {
        return Story.getList({});
      }
    },
    stories: {
      get: function(last, resolve) {
        this.attr("storyPromise").then(resolve);
      }
    },
    sum2: {
      get: function() {
        var total = 0;
        if (this.attr('stories') !== undefined) {
          this.attr('stories').each(function(elem) {
            total += parseInt( elem.attr("pages") );
          });
        }
        return total;
      }
    }
  }
});

You have to use .attr, otherwise, sum2 doesn't know when the pages property changes.