group_reduce.js

var group_reduce = function (dependentKey, key_func, reduce) {
  return Ember.reduceComputed.call(null, dependentKey, {
    initialValue: Ember.A,
    initialize: function(initialValue, changeMeta, instanceMeta) {
      instanceMeta.meta = new Ember.Map();
    },
    addedItem: function (accumulatedValue, obj, changeMeta, instanceMeta) {
      var key = key_func(obj), reduced_object;

      if (instanceMeta.meta.get(key) === undefined) {
        instanceMeta.meta.set(key, {items: Ember.A(), index: accumulatedValue.get('length')});
      }

      instanceMeta.meta.get(key).items.push(obj);

      reduced_object = reduce(key, instanceMeta.meta.get(key).items);

      accumulatedValue.replace(instanceMeta.meta.get(key).index, 1, [reduced_object]);

      return accumulatedValue;
    },
    removedItem: function (accumulatedValue, obj, changeMeta, instanceMeta) {
      var key = null, keyfound = false, index;


      instanceMeta.meta.forEach(function(i, val) {
        if (val.items.contains(obj)) {
          key = i;
          keyfound = true;
        }
      });

      if ( ! keyfound )
        throw new Error("You just asked me to remove an object " +
                        "that's not in instanceMeta");

      index = instanceMeta.meta.get(key).index;
      instanceMeta.meta.get(key).items.removeAt(
        instanceMeta.meta.get(key).items.indexOf(obj));

      if (instanceMeta.meta.get(key).items.length == 0) {
        accumulatedValue.removeAt(index);

        instanceMeta.meta.forEach(function(i, val) {
          if (val.index > index)
            val.index--;
        });

        instanceMeta.meta.remove(key)
      }
      else {
        accumulatedValue.replace(index, 1,
          [reduce(key, instanceMeta.meta.get(key).items)]
        );
      }

      return accumulatedValue;
    }
  });
}