MongoDB, MapReduce and sorting

I might be a bit in over my head on this as I'm still learning the ins and outs of MongoDB, but here goes.

Right now I'm working on a tool to search/filter through a dataset, sort it by an arbitrary datapoint (eg. popularity) and then group it by an id. The only way I see I can do this is through Mongo's MapReduce functionality.

I can't use .group() because I'm working with more than 10,000 keys and I also need to be able to sort the dataset.

My MapReduce code is working just fine, except for one thing: sorting. Sorting just doesn't want to work at all.

db.runCommand({
  'mapreduce': 'products',
  'map': function() {
    emit({
      product_id: this.product_id,
      popularity: this.popularity
    }, 1);
  },
  'reduce': function(key, values) {
    var sum = 0;
    values.forEach(function(v) {
      sum += v;
    });

    return sum;  
  },
  'query': {category_id: 20},
  'out': {inline: 1},
  'sort': {popularity: -1}
});

I already have a descending index on the popularity datapoint, so it's definitely not working because of a lack of that:

{ 
  "v" : 1, 
  "key" : { "popularity" : -1 }, 
  "ns" : "app.products", 
  "name" : "popularity_-1" 
}

I just cannot figure out why it doesn't want to sort.

Instead of inlining the result set, I can't output it to another collection and then run a .find().sort({popularity: -1}) on that because of the way this feature is going to work.