why transform.options auto convert inline params model to json ?

[smallst]

Do you want to request a feature or report a bug?
i'm not sure it is a bug or feature, but the behaviors change since mongoose 4 -> 5/6. and without any breaking changelogs.
What is the current behavior?
for now, if you define the transform function for a schema, the inline params passed into options no longer support mongoose model. (but supports in mongoose 4). so you could not call options.model.get anymore.

If the current behavior is a bug, please provide the steps to reproduce.

here is a gist to reproduce the behaviors .
https://gist.github.com/smallst/506f493bbe47f5bf1d62d1485b88d12c
see line 34 and 14-17.

and here is the running results

What is the expected behavior?
i think if we pass in a model, it should be still a model inside transform function. Or you could tell me (in the Doc) why it is not.
What are the versions of Node.js, Mongoose and MongoDB you are using? Note that "latest" is not a version.
version:
nodejs: v17.9.0
mongoose: 6.3.2
mongodb: 4.0

[IslandRhythms]

const mongoose = require('mongoose')
const Schema = mongoose.Schema;
const ObjectId = Schema.ObjectId;

const ASchema = new Schema({
  name: String
})
const BSchema = new Schema({
  name: String,
  aName: String
})

ASchema.set('toObject', {
  transform: (doc, ret, options) => {
    console.log('correct:', options.b.aName)         // now this is the correct way
    console.log('error:', options.b.get('aName'))    // in mongoose 4 we use this.
  }
})

const AModel = mongoose.model('AModel', ASchema)
const BModel = mongoose.model('BModel', BSchema)

let main = async () => {
  await mongoose.connect('mongodb://localhost/test_mongoose');
  await mongoose.connection.dropDatabase();
  const a = new AModel()
  a.name = 'a'
  await a.save()

  const b = new BModel()
  b.name = 'b';
  b.aName = 'a';
  await b.save()

  console.log(a.toObject({b}))               // pass in a model, but no longer get a model

}

main().then(() => {console.log('done')})

[smallst]

i think the bug comes from here and here, you return the ._doc when clone mongoose model and this clone called on transform options.

and in mongoose4 you don't clone such options so goes well

[smallst]

i don't know if utils.clone should work as clone mongoose model to normal object (._doc) , and if utils.clone returns mongoose model as model would cause any issue. but maybe another utils.cloneModel function can work here? which clone everything as everything.
not sure if this suggests helps.
and i'm still using mongoose 5 (just upgraded from mongoose 4) in my company, that's a huge codebase so maybe not so easy to upgrade to mongoose 6 soon, if you fix this bug on mongoose 6 could you also publish a new version of mongoose 5 with same bugfix? thanks a lot

[vkarpov15]

Count this as another use case for #11849. We'll use shallow clone because there's no reason for us to deep clone options.

[smallst]

thanks!