title: Re-initialize template with new id on selectbox change
body: <p>I have a selectbox in template A which selects an item "name and id".</p>
  
  <p>I also have an "item" template that needs an ID as parameter to load its data from its database. I am using a session variable for the id and I pass the id to the "item" template using return Session.get . This only works for on load. When the session variable change the "item" template is not updated. How do I get the "item" template to re-initialize</p>
  
  <p>Some code:</p>
  
  <pre><code> Template.selectBox.helpers({
      selectList: function () {
          return Templates.find({}, {fields: {'_id': 1, 'name': 1}});
      },
      selectedId: function() {
          return Session.get("selectedId");
      }
  });
  
  Template.selectBox.events({
      'change #item-chooser': function (event) {
          var selectedId = $(event.currentTarget).find(":selected").val();
          if (typeof(selectedId) === 'undefined'
              || selectedId === "new") {
              Session.set("selectedId", "new");
          }
          else {
              Session.set("selectedId", selectedId);
          }
      }
  });
  </code></pre>
  
  <p>The items template is called using</p>
  
  <pre><code>        {{&gt; item selectedId}}
  
  Template.item.onCreated(function() {
      var selectedId = this.data.selectedId;
      this.selectedItem = new ReactiveVar;
      if (typeof(selectedId) === 'undefined'
          || selectedId === "new") {
          this.selectedItem.set(emptyItem);
      }
      else {
          var selectedItemData = Templates.findOne({_id: selectedId});
          this.selectedItem.set(selectedItemData );
      }
  });
  </code></pre>