code: function(username, password, callback) {
  
              var args = Array.prototype.slice.call(arguments);
  
              if (args.length === 1) {
                  callback = arguments[0];
              } else {
                  this.set({
                      username: username,
                      password: password
                  });
              }
  
              this.validateAndSave(null, function(err, session) {
                  if (!err && session) {
                      session.set("password", null);
  
                      session.fetch({
                          fetchSubModels: ["user"]
                      }, callback);
                  } else {
                      callback(err);
                  }
              });
          }
docs: *
  
  @param [username]
  @param [password]
  @param callback