.toObject({flattenMaps: true}) on parent requires explicitly setting flattenMaps: true on child documents nested in maps

[TimUnderhay]

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

What is the current behavior?
When calling .toObject({flattenMaps: true}) on a parent document, maps of child documents are not flattened, unless one explicitly sets childSchema.set('toObject', { flattenMaps: true }); on every child schema.

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

const mongoose = require('mongoose');
const util = require('util');

const NestedChildSchema = new mongoose.Schema({
  myValue: {
    type: String
  }
}, {_id: false});

const ChildSchema = new mongoose.Schema({
  myChildMap: {
    type: Map,
    of: NestedChildSchema
  }
}, {_id: false});


const Schema = new mongoose.Schema({
  myMapSubdocs: {
    type: Map,
    of: ChildSchema,
    required: true
  }
}, {minimize: false, collection: 'test'});

const test = async () => {
  const nestedChild = {myValue: 'abc'};
  const child = {
    myChildMap: new Map().set('myChildMap', nestedChild)
  };
  const parent = {
    myMapSubdocs: new Map().set('myChild', child)
  };
  const BadModel = mongoose.model('BadModel', Schema);
  const badParentDoc = new BadModel(parent);

  const myBadFlattenedObject = badParentDoc.toObject({flattenMaps: true});
  console.log(`Doesn't flatten child schemas when they're part of maps:`)
  console.log(util.inspect(myBadFlattenedObject, {showHidden: false, depth: null}));
    
  console.log(`\n\nProperly flattens child schemas when they're part of maps.  The only difference is we set flattenMaps on ChildSchema:`)
  ChildSchema.set('toObject', { flattenMaps: true });
  const GoodModel = mongoose.model('GoodModel', Schema);
  const goodParentDoc = new GoodModel(parent);
  const goodFlattenedObject = goodParentDoc.toObject({flattenMaps: true});
  console.log(util.inspect(goodFlattenedObject, {showHidden: false, depth: null}));
};

const main = async () => {
  await test();
  process.exit(0);
};

main();

Output:

Doesn't flatten child schemas when they're part of maps:
{
  myMapSubdocs: {
    myChild: { myChildMap: Map(1) { 'myChildMap' => { myValue: 'abc' } } }
  },
  _id: new ObjectId("612f9f9ff10699bd21543208")
}


Properly flattens child schemas when they're part of maps.  The only difference is we set flattenMaps on ChildSchema:
{
  myMapSubdocs: {
    myChild: { myChildMap: { myChildMap: { myValue: 'abc' } } }
  },
  _id: new ObjectId("612f9f9ff10699bd21543209")
}

What is the expected behavior?
If I call .toObject({flattenMaps: true}) on a parent document, I would expect not to have to explicitly set flattenMaps: true on all child schemas which are part of maps in order to flatten them.

What are the versions of Node.js, Mongoose and MongoDB you are using? Note that "latest" is not a version.
Mongoose 6.0.3
Node v14.17.6

[IslandRhythms]

I'm getting the same output

const mongoose = require('mongoose');
const util = require('util');

const NestedChildSchema = new mongoose.Schema({
  myValue: {
    type: String
  }
}, {_id: false});

const ChildSchema = new mongoose.Schema({
  myChildMap: {
    type: Map,
    of: NestedChildSchema
  }
}, {_id: false});


const Schema = new mongoose.Schema({
  myMapSubdocs: {
    type: Map,
    of: ChildSchema,
    required: true
  }
}, {minimize: false, collection: 'test'});

const test = async () => {
    await mongoose.connect('mongodb://localhost:27017/test', {
        useNewUrlParser: true,
        useUnifiedTopology: true,
      });
      await mongoose.connection.dropDatabase();
  const nestedChild = {myValue: 'abc'};
  const child = {
    myChildMap: new Map().set('myChildMap', nestedChild)
  };
  const parent = {
    myMapSubdocs: new Map().set('myChild', child)
  };
  const BadModel = mongoose.model('BadModel', Schema);
  const badParentDoc = new BadModel(parent);

  const myBadFlattenedObject = badParentDoc.toObject({flattenMaps: true});
  console.log(`Doesn't flatten child schemas when they're part of maps:`)
  console.log(util.inspect(myBadFlattenedObject, {showHidden: false, depth: null}));
    
  console.log(`\n\nProperly flattens child schemas when they're part of maps.  The only difference is we set flattenMaps on ChildSchema:`)
  ChildSchema.set('toObject', { flattenMaps: true });
  const GoodModel = mongoose.model('GoodModel', Schema);
  const goodParentDoc = new GoodModel(parent);
  const goodFlattenedObject = goodParentDoc.toObject({flattenMaps: true});
  console.log(util.inspect(goodFlattenedObject, {showHidden: false, depth: null}));
};

const main = async () => {
  await test();
  process.exit(0);
};

main();