Computed groupBy

groupBy.js

(function() {
  var get = Ember.get, set = Ember.set;

  function findItemInsertionIndex(group, changeMeta, instanceMeta) {
    for (var i = get(group, 'length') - 1; i >= 0; i--) {
      var currentItem = group.objectAt(i),
          currentIndex = instanceMeta.itemGuidToIndex[Ember.guidFor(currentItem)];

      if (currentIndex < changeMeta.index) {
        return i + 1;
      }
    }

    return 0;
  }

  function getGroup(groups, item, propertyKey) {
    var value = get(item, propertyKey);
    var group = groups.findBy('value', value);

    if (!group) {
      group = Ember.A();
      set(group, 'value', value);
      groups.pushObject(group);
    }

    return group;
  }

  function removeItem(groups, group, item) {
    group.removeObject(item);

    if (Ember.isEmpty(group)) {
      groups.removeObject(group);
    }
  }

  Ember.computed.groupBy = function(dependentKey, propertyKey) {
    return Ember.reduceComputed(dependentKey + ".@each." + propertyKey, {

      initialize: function (groups, changeMeta, instanceMeta) {
          instanceMeta.itemGuidToIndex = Ember.Object.create();
          return groups;
        },

      initialValue: function() {
        return Ember.A();
      },

      addedItem: function(groups, item, changeMeta, instanceMeta) {
        instanceMeta.itemGuidToIndex[Ember.guidFor(item)] = changeMeta.index;

        var group = getGroup(groups, item, propertyKey);
        var insertionIndex = findItemInsertionIndex(group, changeMeta, instanceMeta);
        group.insertAt(insertionIndex, item);

        return groups;
      },

      removedItem: function(groups, item, changeMeta, instanceMeta) {
        var value;

        if (changeMeta.previousValues) {
          value = changeMeta.previousValues[propertyKey];
        } else {
          delete instanceMeta.itemGuidToIndex[Ember.guidFor(item)];
          value = get(item, propertyKey);
        }

        var group = groups.findBy('value', value);
        removeItem(groups, group, item);

        return groups;
      }

    });
  };
})();

Example: http://emberjs.jsbin.com/IpeHuWi/2/edit

Takes as input an array of objects and a key. Ouputs an array of arrays grouped by value.