What is the correct way to use next-auth with Google OAuth and users stored in the database? - next.js

Question

I'm new to next-auth, and I'm looking for some help.
I have added the Google OAuth provider, and now when I run signIn("google") function on the frontend, it automatically takes me to the google's login page, and logs me in, somehow, without ever touching my database.
When the google authentication is complete, I need to be able to create a new user in my database, or retrieve the existing one if they have already signed up before (because I need to store all kinds of custom information about the user, not just their email).
And I want to make user's information available on the session object from useSession()hook. Right now I'm seeing some kind of default user info (with name, email, and image field which I didn't define).
When I was using a regular Express server and Passport, the code looked kinda like this:
const googleAuth = new GoogleStrategy(
{
clientID: process.env.GOOGLE_CLIENT_ID,
clientSecret: process.env.GOOGLE_CLIENT_SECRET,
callbackURL: "/api/v1/profiles/google/callback",
},
async (req, accessToken, refreshToken, profile, done) => {
const existingProfile = await Profile.findOne({ email: profile.emails[0].value })
/* Found user, return him and move along. */
if (existingProfile) return done(null, existingProfile)
/* Haven't found profile with this googleId, create a new one. */
const newProfile = await new Profile({
googleId: profile.id,
email: profile.emails[0].value,
})
newProfile.save()
done(null, newProfile)
}
)
So I would still be creating the users in my database, and retrieving their information on log in, so that I could send it to the client.
Where does this kind of code supposed to go when I'm using the serverless next-auth?
And a second, but kind of related question - what's that default user object that gets provided to me in the session object? The one with the name, email, and image fields that next-auth seems to create for me? How can I make it use the user object I'm returning from my database instead?
(I've done my best to look through the tutorials and examples, but couldn't find one that explains this clearly.)

Answer 1

I don't know if you still need this, but I hope it helps someone:
Oauth kinda mixes up Sign In and Sign Up, so if you want to have Google authentication what you probably want to do is create a callback of the Sign In function in /api/auth/[...nextauth].js, then get the account and profile as parameters and access to its provider.
async signIn({account, profile}) {
if(account.provider === 'google') {
//check if user is in your database
if(user NOT in DB) {
//add your user in DB here with profile data (profile.email, profile.name)
}
return true
}
You always want to return true since you always want to log in independently if it is in your DB or not.
Regarding the session object, you can also add a callback and access to the default session (that you can modify), token and user. Here you can retrieve all information you want from your database, add it to the session object and return it.
async session({ session, token, user }) {
const newData = DB.find(...).data
session.newfield = newInfo
return session
}