Grouping Breaks Reactivity

gistfile1.js

// collections/assignments.js
Deps.autorun(function(){
  Assignments.grouped = {};
  var raw_list = Assignments.find().fetch();
  for (var i = raw_list.length - 1; i >= 0; i--) {
    if (Assignments.grouped[raw_list[i].date] == undefined) {Assignments.grouped[raw_list[i].date]={}};
    if (Assignments.grouped[raw_list[i].date][raw_list[i].meetingId] == undefined) {Assignments.grouped[raw_list[i].date][raw_list[i].meetingId]=[]};
    Assignments.grouped[raw_list[i].date][raw_list[i].meetingId].push(raw_list[i]);
  };
});

// views/assignments.js
Template.assignments.helpers({
  meetings: function(){
    return Assignments.grouped[Session.get('current_date')];
  },
});