Error when access the _id from mongodb nextjs auth when login and returning session user

Question

I was following a tutorial to create a blogging site. I used Nextauth with Google and MongoDB to sign up. When a user is not signed in, I could sign up with Google and create a user in mongoDB. I can see the records in the database and each of them has _id, user, email..... I log in after the signup but when trying to get the _id from the mongoDB and assign it to the session user it fails.

The following statement is producing an error it says that _id is undefined

session.user.id = sessionUser._id.toString();

return session

This is the full block of code

import NextAuth from 'next-auth';
import GoogleProvider from 'next-auth/providers/google';

import User from '@models/user';
import { connectToDB } from '@utils/database';

const handler = NextAuth({
  providers: [
    GoogleProvider({
      clientId: process.env.GOOGLE_ID,
      clientSecret: process.env.GOOGLE_CLIENT_SECRET,
    })
  ],
  callbacks: {
    async session({ session }) {
      // store the user id from MongoDB to session
      const sessionUser = await User.findOne({ email: session.user.email });

      session.user.id = sessionUser._id.toString();

      return session;
    },
    async signIn({ account, profile, user, credentials }) {
      try {
        await connectToDB();

        // check if user already exists
        const userExists = await User.findOne({ email: profile.email });

        // if not, create a new document and save user in MongoDB
        if (!userExists) {
          await User.create({
            email: profile.email,
            username: profile.name.replace(" ", "").toLowerCase(),
            image: profile.picture,
          });
        }

        return true
      } catch (error) {
        console.log("Error checking if user exists: ", error.message);
        return false
      }
    },
  }
})

export { handler as GET, handler as POST }

Answer 1

it's because there is no user with such email in your database or the part

User.findOne({ email: session.user.email })

is somehow not doing what it is supposed to do I would recommend doing something like

also check if email is the right field in your database there is a slight chance you might have mistyped it

import NextAuth from 'next-auth';
import GoogleProvider from 'next-auth/providers/google';

import User from '@models/user';
import { connectToDB } from '@utils/database';

const handler = NextAuth({
    providers: [
        GoogleProvider({
            clientId: process.env.GOOGLE_ID,
            clientSecret: process.env.GOOGLE_CLIENT_SECRET,
        })
    ],
    callbacks: {
        async session({ session }) {
            // store the user id from MongoDB to session
            // log the email to make sure you have the right annotations since you are not using typescript | you may delete this in future
            console.log(session.user.email)
            const sessionUser = await User.findOne({ email: session.user.email });
            if (sessionUser) {
                session.user.id = sessionUser._id.toString();
                return session;                
            } else {
                // no such user Handle it as your application needs
            }

        },
        async signIn({ account, profile, user, credentials }) {
            try {
                await connectToDB();

                // check if user already exists
                const userExists = await User.findOne({ email: profile.email });

                // if not, create a new document and save user in MongoDB
                if (!userExists) {
                    await User.create({
                        email: profile.email,
                        username: profile.name.replace(" ", "").toLowerCase(),
                        image: profile.picture,
                    });
                }

                return true
            } catch (error) {
                console.log("Error checking if user exists: ", error.message);
                return false
            }
        },
    }
})

export { handler as GET, handler as POST }