mongoose timestamps createdAt added to existing documents upon doc.save()

[guy-evdev]

nodejs version 14.16.0
mongoose version 5.12.5
MongoDB driver (nodejs) 3.6.5

mongoose timestamps configuration:
{timestamps: {createdAt: 'createdAt', updatedAt: 'lut'}}

Steps for reproducing:

1. Document without the createdAt field (was created before adding timestamps to the schema).
2. Document retrieved from DB using findOne() with no field projection
3. Adding/modifying fields
4. Saving the document using doc.save()
   After saving, the createdAt field is added to the document with the current date which of course is not the expected behavior since the createdAt field should reflect the actual document creation date (which was created much earlier)

It will not happen when:
Using findAndUpdate (much better the findOne and save - I know)
Using findOne with field projection

As a temporary workaround, I added the following to all schemas using timestamps
createdAt: {type: Date, select: false}
When the above is added, the createdAt field is not added to the existing document upon saving but this is definitely intended as a quick-fix and not a permanent solution since the expected behaviour is that the createdAt field will not be added to an existing document if not specifically requested.

[IslandRhythms]

The only field being added is the updatedAt timestamps when you include the schema on mongoose 6.0.13
This bug is reproducable in your version of mongoose, 5.12.5
@vkarpov15 How do you want to proceed?

const mongoose = require('mongoose');
const {Schema} = mongoose;
/*
const testSchema = new Schema({
    name: String
});
*/

const testSchema = new Schema({
    name: String
}, {timestamps: {createdAt: 'createdAt', updatedAt: 'lut'}});

const Test = mongoose.model('Test', testSchema);

async function run() {
    await mongoose.connect('mongodb://localhost:27017/', {useNewUrlParser: true,
    useUnifiedTopology: true,});
    // await mongoose.connection.dropDatabase();
    // await Test.create({name: 'Test Testerson'});
    const entry = await Test.findOne();
    entry.name = 'Modified Timestamp names';
    await entry.save();
    console.log(await Test.find())
}

run();

[vkarpov15]

This issue is fixed in 6.0.0. But still affects all 5.x versions. Please upgrade to 6.0.0 - we'll work on a fix for the 5.x branch.