Why do the insertOne method from mongodb node.js driver mutates the object to be inserted?

Question

I am learning mongodb with node, and I was playing with the following code

 var assert = require('assert') var url = 'mongodb://localhost:27017/learnyoumongo' var client = require('mongodb').MongoClient var doc = { firstName: 'Steve', lastName: 'Smith' } console.log(doc) //logs as expected client.connect(url, (err, db) => { assert.equal(err, null) var docs = db.collection('docs') docs.insertOne(doc, (err, result) => { assert.equal(err, null) console.log(doc) //logs with an extra property ie _id db.close() }) }) 

I was surprised to see that doc is mutated by mongo, look at inspect the output of both of the console.log statements. Why is the doc object mutated.

Answer 1

Mongo adds an automatically generated _id to every document that doesn't define one itself. This is a special object type called an ObjectId and is used as a primary key. You can see the details of the format here .

You can get around the auto-generated _id by adding your own to each object but you'll need to be able to guarantee that they're unique as if you try to store two objects with the same _id you'll get a duplicate key error.