{ optimisticConcurrency: true }, unwanted rollback of __v

[sebamisc]

Environment
"mongoose": "^5.11.17"

Working with the option { optimisticConcurrency: true } (overwriting versionKey does not affect behavior)

High-Level Description
When relying on optimisticConcurrency to manage the versionKey, I noticed a bug, causing the versionKey to roll back.

Schema Description
The following schema was used for reproduction:

const thingSchema = new mongoose.Schema(
  {
    price: {
      type: Number
    },
  }, 
  {
    optimisticConcurrency: true,
  }
);

const Thing = mongoose.model('Thing', thingSchema);

Testing Steps
The following script was used to cause the rollback:

const thing = new Thing.create({ price: 1 });
await thing.save();
console.log(thing);
// prints out __v: 0

// get thing 1st time
const thing_1 = await Thing.findById(thing.id);

// get thing 2nd time
const thing_2 = await Thing.findById(thing.id);

// manipulate the things
thing_1.set({price: 2});
await thing_1.save();  // persist correctly __v: 1

thing_2.set({price: 1});
await thing_2.save();  // persists rollback to  __v: 0

Expected
An error on thing_2.save();, however operation went through sucessfully and reset version to __v:0.

Note
When setting thing_2 dirty 'forceably' , error is thrown, as such:

thing_2.set({price: 2});
thing_2.set({price: 1});
await thing_2.save();  // throws!

Manual workaround
The only manual workaround for now seems to be to set

thingSchema.pre('save', async function (done) {
 this.increment();
 done()
});

[IslandRhythms]

const mongoose = require('mongoose');

const run = async () => {
    await mongoose.connect('mongodb://localhost:27017/test', {
      useNewUrlParser: true,
      useUnifiedTopology: true,
    });
    await mongoose.connection.dropDatabase();
    const thingSchema = new mongoose.Schema({price: Number},{optimisticConcurrency: true});
    const Thing = mongoose.model('Thing', thingSchema);
    const thing = await Thing.create({price: 1});
    await thing.save();
    console.log(thing);
    const thing_1 = await Thing.findById(thing.id);
    const thing_2 = await Thing.findById(thing.id);
    thing_1.set({price: 2});
    await thing_1.save();
    console.log(thing_1);
    thing_2.set({price:1});
    await thing_2.save();
    console.log(thing_2);

}
run().catch(err => console.log(err));