TopicStatistics.methods.createTopicStatistics = function(callback)
    {
        // do stuff

        Topic.find({} , function (err, resultList)
        {
             if(err)
             {
                 // do stuff
                 if(typeof callback === "function")
                    callback(err); // callback undefined
             }
             else
             {
                 // do stuff
                 if(typeof callback === "function")
                    callback(null); // callback undefined
             }

        });
    };

Topic.find({} , function (err, resultList, mycallback);