code: function joinCurrentModelActivity(method, stateName, modelOrCollection, context, force) {
          var xhrActivity = modelOrCollection[xhrModelLoadingAttribute];
          if (xhrActivity) {
              _.each(xhrActivity, function(xhrEvent) {
                  if (!method || method === ALL_XHR_ACTIVITY || xhrEvent.method === method) {
                      // this is one that is applicable
                      pushLoadingState(xhrEvent, stateName, modelOrCollection, context, force);
                  }
              });
          }
      }
docs: if there is any current xhrEvent that match the method, add a reference to it with this context