duplicate key error index in mongodb mongoose - node.js

Question

I am building a website in which logging in with Google and typing email and password were working fine, but when I introduced logging in with Facebook, mongoDb is giving following error-
MongoError: E11000 duplicate key error collection: userDB.users index: username_1 dup key: { username: null }
//jshint esversion:6
require('dotenv').config();
const express = require("express");
const bodyParser = require("body-parser");
const ejs= require("ejs");
const mongoose = require("mongoose");
const session = require("express-session");
const passport = require("passport");
const passportLocalMongoose = require("passport-local-mongoose");
const GoogleStrategy = require('passport-google-oauth20').Strategy;
const FacebookStrategy = require('passport-facebook').Strategy;
const findOrCreate = require("mongoose-findorcreate");
const app = express();
console.log(process.env.API_KEY);
app.use(express.static("public"));
app.set('view engine', 'ejs');
app.use(bodyParser.urlencoded({
extended: true
}));
app.use(session({
secret: "Our little secret.",
resave: false,
saveIninitialized: false
}));
app.use(passport.initialize());
app.use(passport.session());
mongoose.connect("mongodb://localhost:27017/userDB",{ useUnifiedTopology: true });
mongoose.set("useCreateIndex", true);
const userSchema = new mongoose.Schema ({
email: String,
password: String,
googleId: String,
facebookId: String,
secret: String
});
userSchema.plugin(passportLocalMongoose);
userSchema.plugin(findOrCreate);
const User = new mongoose.model("User", userSchema);
passport.use(User.createStrategy());
passport.serializeUser(function(user, done) {
done(null, user.id);
});
passport.deserializeUser(function(id, done) {
User.findById(id, function(err, user) {
done(err, user);
});
});
passport.use(new GoogleStrategy({
clientID: process.env.CLIENT_ID,
clientSecret: process.env.CLIENT_SECRET,
callbackURL: "http://localhost:3000/auth/google/secrets",
userProfileURL: "https://www.googleapis.com/oauth2/v3/userinfo"
},
function(accessToken, refreshToken, profile, cb) {
User.findOrCreate({ googleId: profile.id }, function (err, user) {
return cb(err, user);
});
}
));
passport.use(new FacebookStrategy({
clientID: process.env.FACEBOOK_APP_ID,
clientSecret: process.env.FACEBOOK_APP_SECRET,
callbackURL: "http://localhost:3000/auth/facebook/secrets"
},
function(accessToken, refreshToken, profile, cb) {
User.findOrCreate({ facebookId: profile.id }, function (err, user) {
return cb(err, user);
});
}
));
app.get("/",function(req,res){
res.render("home")
});
app.get('/auth/google',
passport.authenticate('google', { scope: ['profile'] })
);
app.get('/auth/google/secrets',
passport.authenticate('google', { failureRedirect: '/login' }),
function(req, res) {
// Successful authentication, redirect to secrets.
res.redirect('/secrets');
});
app.get('/auth/facebook',
passport.authenticate('facebook'));
app.get('/auth/facebook/secrets',
passport.authenticate('facebook', { failureRedirect: '/login' }),
function(req, res) {
// Successful authentication, redirect home.
res.redirect('/secrets');
});
app.get("/login",function(req,res){
res.render("login")
});
app.get("/register",function(req,res){
res.render("register")
});
...
How to solve this issue so that I can login via both google and facebook one after the other or vice-versa?

Answer 1

see the solution here:
Setting Up Facebook Authentication with MongoDB Atlas and Passport.js
the problem is with the code above that both facebook and google users create a document in database which has no username. and MongoD set username as unique by default. so just drop the indexes from username and that solves the issue.