JSON inserted into mongodb comes out as a string

I'm trying to insert a json, which originates from a POST request, as a document into a mongodb collection. I'm using node.js and Express 4.

My code:

const express = require('express');
const app = express();
const bodyParser = require('body-parser');
const request = require('request');

var MongoClient = require('mongodb').MongoClient
    , assert = require('assert');

app.use(bodyParser.urlencoded({ extended: false, limit: '50mb' }));
app.use(bodyParser.json({ limit: '50mb' }));

var insertReading = function(datum, db) {
  var collection = db.collection('readings');
  collection.insertOne(JSON.parse(datum), function(err, r) {
    assert.equal(err, null);
    assert.equal(1, r.insertedCount);
  });
}

app.post('/save/', function (req, res) {
  console.log(req.query);
  console.log(req.body);

  MongoClient.connect(url, function(err, db) {
    assert.equal(null, err);
    insertReading(req.body), db);
    db.close();
  });
  res.send('All done!');
});

When looking at the created document, it looks like that:

> for (var key in db.readings.findOne()) {print(key, typeof key);}
_id string
payload string

I'd expect the payload to be a whole document, with more keys in it etc. but it is just a string.

I can later convert this payload into a document:

db.readings.find().forEach(function(myDoc) {db.parsed.insertOne(JSON.parse(myDoc['payload']));});

but I would like to just get it right on the first insertion...

I already tried adding JSON.parse to the node.js code, but it does not help, though on the shell command it does the trick...

What am I doing wrong? Is there a function I can't find that turns a json string into a document upon insertion?

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