// @/database/index.ts

import { connect } from 'mongoose'

export default async function () {
    return connect ('mongodb+srv://not:going@to.reveal.my.url/?retryWrites=true&w=majority')
}

// @/database/User.ts

import { Schema, model as Model } from 'mongoose'

const schema = new Schema ({
    // ...
}),
    model = Model ('user', schema)

export default model

// @/app/api/auth/route.tsx

import { NextRequest, NextResponse } from 'next/server'

import connect from '@/database'
import User from '@/database/User'

export async function POST (request: NextRequest) {
    await connect ()
    
    // ...

    const foundUser = await User.findOne ({ id: user.id })

    if (foundUser) return NextResponse.json (foundUser)

    const newUser = new User ({
        // ...
    }),
          savedNewUser = await newUser.save ()

    return NextResponse.json (savedNewUser)
}

- error Error [OverwriteModelError]: Cannot overwrite `user` model once compiled.