code: function (query, callback) {
      if (!callback) {
        callback = query;
        query = null;
      }
  
      if (typeof query === 'string') {
        query = { aggregateId: query };
      }
  
      this.store.getLastEvent(query, callback);
    }
docs: loads the last event
  @param {Object || String} query    the query object [optional]
  @param {Function}         callback the function that will be called when this action has finished
  `function(err, event){}`