`nModified: 1` returned by `.update()` when $addToSet makes no modification

current behavior

User with id 1 has foo: ['bar'] currently in db. I run:

User.update({_id: 1}, {$addToSet: {foo: 'bar'}}).exec().then(res => {
   console.log(res)
}

And it outputs { n: 1, nModified: 1, ok: 1 }. But the document was not actually modified since it did not add anything to foo, as bar was already in the array.

expected behavior

I want to be able to check whether foo array already contained ‘bar’, as this determines whether I continue. In a more concrete example, if the user gets points for doing an action, and completed actions are stored in an array, I want to be able to check if the user had already taken the action so I can avoid giving them points. I am aware I could just fetch the user from the db and check, but then if a user hits the api with concurrent requests using a script, they could get points multiple times. The $addToSet method with a check for updated or not seems like a great atomic way to do this, but it always returns nModified: 1 so I am unable to currently.

Versions Mongoose 4.13.9, MongoDB 3.6.2

[RobinJayaswal]

@lineus i do in fact have timestamps on. I suppose I expected the timestamps to only update in a real update, but now that I think about it they are probably implemented in a pre hook.

[lineus]

@RobinJayaswal do you have timestamps turned on by chance? or something else that might be updating a path in the document besides what your example shows? I tried to replicate what you’re seeing with the following code, but it doesn’t show the same behavior:

6861.js

#!/usr/bin/env node
'use strict';

const assert = require('assert');
const mongoose = require('mongoose');
mongoose.Promise = global.Promise;
mongoose.connect('mongodb://localhost:27017/test', { useMongoClient: true });
const conn = mongoose.connection;
const Schema = mongoose.Schema;

const schema = new Schema({
  foo: [String]
});

const Test = mongoose.model('test', schema);

const test = new Test({ foo: ['bar'] });

async function run() {
  await conn.dropDatabase();
  await test.save();
  let res = await Test.update({ _id: test._id }, { $addToSet: { foo: 'bar'} });
  assert.strictEqual(res.nModified, 0);
  console.info(`Test passed on ${mongoose.version}`);
  return conn.close();
}

run();

Output:

issues: ./6861.js
Test passed on 4.13.9
issues: