`document#populate` behaves differently with `query#populate` in mongoose 4.x

[bbqaaq]

Do you want to request a feature or report a bug?
Probably a bug.

What is the current behavior?
In the latest version of mongoose 4.x,document#populate behaves differently with query#populate

If the current behavior is a bug, please provide the steps to reproduce.
When using the query#populate, the document's populated field can be modified normally:

let potter = await Book.findOne({ name: 'Potter' }).populate('owner').exec()
console.log('before potter = ', potter)
potter.owner = {}
console.log('after potter = ', potter)

and the result is this

before potter =  { _id: 5c08b95a0590e665915a46d1,
  name: 'Potter',
  owner: 
   { _id: 5c08b95a0590e665915a46cf,
     name: 'Peter',
     age: '12',
     __v: 0 },
  __v: 0 }
after potter =  { _id: 5c08b95a0590e665915a46d1,
  name: 'Potter',
  owner: { _id: 5c08cd6227b7516865831774 },
  __v: 0 }

However, when using document#populate, the owner field will become unchangeable unless the new value is a mongoose document or null

let potter = await Book.findOne({ name: 'Potter' }).exec()
await potter.populate('owner').execPopulate()
console.log('before potter = ', potter)
potter.owner = {}
console.log('after potter = ', potter)

and the result is this

before potter =  { _id: 5c08b95a0590e665915a46d1,
  name: 'Potter',
  owner: 
   { _id: 5c08b95a0590e665915a46cf,
     name: 'Peter',
     age: '12',
     __v: 0 },
  __v: 0 }
after potter =  { _id: 5c08b95a0590e665915a46d1,
  name: 'Potter',
  owner: 
   { _id: 5c08b95a0590e665915a46cf,
     name: 'Peter',
     age: '12',
     __v: 0 },
  __v: 0 }

This problem isn't appear when I was still using mongoose 4.7.8, it just appear when I try to push the mongoose version to latest 4.x. For mongoose 5.x, this problem don't appear too.

Please mention your node.js, mongoose and MongoDB version.
Node 8.12.0
Mongoose 4.13.17
MongoDB 3.2.21

[vkarpov15]

Thanks for reporting, will investigate ASAP

[bbqaaq]

Thanks @vkarpov15
You make it as "needs repro script"? Do I need to provide more information?
I thought my code from the issue is already enough

[vkarpov15]

Thanks for reaching out, the reason for "needs repro script" here is mostly pedantic. For us, a repro means we have either a standalone script or github repo that we can run and confirm that there is an issue. Generally we fill in the below template:

const assert = require('assert');
const mongoose = require('mongoose');
mongoose.set('debug', true);

const GITHUB_ISSUE = `X`;
const connectionString = `mongodb://localhost:27017/${ GITHUB_ISSUE }`;
const { Schema } = mongoose;

run().then(() => console.log('done')).catch(error => console.error(error.stack));

async function run() {
  await mongoose.connect(connectionString);
  await mongoose.connection.dropDatabase();
}

If you're willing to take the time to your code and sample data to the above template, that'll help us move faster on this issue.