问Mongoose + Everyauth奇怪的行为

EN

exports.findOrCreateFacebookUser = function(fbUserData, promise){

  User.findOne({_id:fbUserData.id}, function(err, user) {
    if(err) {
      console.log("Error in finding user for auth. Check Db");
      promise.fail(err);
      return;
    }
    else if(user){
      console.log("User found ");
      promise.fulfill(user);     
    }
    else{

      var joiningUser = new User();
      joiningUser._id = fbUserData.id;
      joiningUser.firstName = fbUserData.first_name;
      joiningUser.lastName = fbUserData.last_name;
      joiningUser.email = fbUserData.email;
      //console.log(JSON.stringify(joiningUser));
      joiningUser.save(function(err){
        if(err){
          console.log("Couldnt save new user: " + err);
          promise.fail(err);
          return;
        }
        else{
          console.log("User wasnt existant, it is now created: " + JSON.stringify(joiningUser));
          promise.fulfill(joiningUser);
        } 
      });
    }
  });
};