Nested Map validation (Map of Map of String) allows any nested value

[headlessme]

Do you want to request a feature or report a bug?
Bug (possibly Feature)

What is the current behavior?
When inserting values into a Map of Map of String, any values is accepted at the innermost level. I'm not sure if this should already be supported or if this actually a feature request.

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

const MySchema = new mongoose.Schema({
  mapOfMapOfString: {
    type: Map,
    of: {
      type: Map,
      of: String
    }
  }
})

const MyModel = mongoose.model('MyModel', MySchema)

const doc = new MyModel()
doc.mapOfMapOfString = { outer: { inner: 123 } }
doc.save()

What is the expected behavior?
Only string values are accepted as values of the nested Map.

What are the versions of Node.js, Mongoose and MongoDB you are using? Note that "latest" is not a version.
Node 14.14.0
Mongoose 6.0.2
MongoDB 4.0.20

[IslandRhythms]

the inner:123 is where an error should be thrown since it is declared to be only strings.

const mongoose = require('mongoose');

const MySchema = new mongoose.Schema({
    mapOfMapOfString: {
      type: Map,
      of: {
        type: Map,
        of: String
      }
    }
  })
  
  const MyModel = mongoose.model('MyModel', MySchema)
  async function test() {
    await mongoose.connect('mongodb://localhost:27017/test', {
        useNewUrlParser: true,
        useUnifiedTopology: true,
      });
      await mongoose.connection.dropDatabase();
    const doc = new MyModel()
    console.log(doc);
    doc.mapOfMapOfString = { outer: { inner: 123 } }
    await doc.save()
    let test = await MyModel.find();
    console.log(test);
    console.log('done');
  }

  test();

[vkarpov15]

The issue here is that Mongoose is interpretting { type: Map, of: { type: Map, of: String } } as { type: Map, of: Map }, so it's losing the nested schema type.

FYI, the correct behavior here is that Mongoose automatically converts number 123 into string '123', which works with d21d2b1