Model.findOneAndUpdate() is updating nested fields with 'strict:false'

[amitpriyankar]

I have observed an unusual behavior when using Model.findOneAndUpdate with a collection that has a flexible schema (strict: false). When I attempt to insert a document that includes a key containing a dot (.), the behavior differs based on whether I define the type of the key in the schema. If I specify the type, the key is treated as a regular string. However, if I don't specify the type, nested fields are created in the document.

Please refer the following code:

const mongoose = require('mongoose');

const url = '<connection_url>;

main().catch((err) => console.log(err));

async function main() {
    await mongoose.connect(url);

    const personSchema = new mongoose.Schema(
        {
            name: String,
            // info: Object,
        },
        {strict: false}
    );

    personSchema.index({name: 1}, {unique: true});

    const Person = mongoose.model('Person', personSchema);

    const savedDoc = await Person.findOneAndUpdate(
        {},
        {name: 'Tommy1', info: {'second.name': 'Shelby'}},
        {new: true, upsert: true}
    )
        .lean()
        .exec();

    console.log('Saved document: ', savedDoc);
    mongoose.disconnect();
}

Saved document:  {
  _id: new ObjectId("646dccd30b5d3da01e058fd0"),
  __v: 0,
  info: { second: { name: 'Shelby' } },
  name: 'Tommy1'
}

I check the doc but didn't find anything which explains this behavior. Could someone please help?

mongoose: v7.1.1
MongoDB Server: 4.0