const fastify = require('fastify')()

fastify.register(require('@fastify/mongodb'), {
  // force to close the mongodb connection when app stopped
  // the default value is false
  forceClose: true,
  
  url: 'mongodb://mongo/mydb'
})

fastify.get('/user/:id', async function (req, reply) {
  // Or this.mongo.client.db('mydb').collection('users')
  const users = this.mongo.db.collection('users')

  // if the id is an ObjectId format, you need to create a new ObjectId
  const id = new this.mongo.ObjectId(req.params.id)
  try {
    const user = await users.findOne({ id })
    return user
  } catch (err) {
    return err
  }
})

fastify.listen({ port: 3000 }, err => {
  if (err) throw err
})

const mongodb = require('mongodb')
mongodb.MongoClient.connect('mongodb://mongo/db')
  .then((client) => {
    const fastify = require('fastify')()

    fastify.register(require('@fastify/mongodb'), { client: client })
      .register(function (fastify, opts, next) {
        const db = fastify.mongo.client.db('mydb')
        // ...
        // ...
        // ...
        next()
      })
  })
  .catch((err) => {
    throw err
  })

const { ObjectId } = require('@fastify/mongodb')

const id = new ObjectId('some-id-here')

const fastify = require('fastify')()

fastify
  .register(require('@fastify/mongodb'), { url: 'mongodb://mongo1/mydb', name: 'MONGO1' })
  .register(require('@fastify/mongodb'), { url: 'mongodb://mongo2/otherdb', name: 'MONGO2' })

fastify.get('/', function (req, res) {
  // This collection comes from "mongodb://mongo1/mydb"
  const coll1 = this.mongo.MONGO1.db.collection('my_collection')
  // This collection comes from "mongodb://mongo2/otherdb"
  const coll2 = this.mongo.MONGO2.db.collection('my_collection')
  // ...
  // ...
  // do your stuff here
  // ...
  // ...
  res.send(yourResult)
})