Document set merge doesn't work for multi layer nested documents

[ianHeydoc]

Prerequisites

• [X] I have written a descriptive issue title
• [X] I have searched existing issues to ensure the bug has not already been reported

Mongoose version

8.6.0

Node.js version

18.15.0

MongoDB server version

NA

Typescript version (if applicable)

No response

Description

When using document set with the merge option set to true if there are multiple nested fields the deeply nested fields are ignored

Steps to Reproduce

const { Schema, model } = require('mongoose');

const personSchema = new Schema({
  info: {
    address: {
      city: String,
      country: { type: String, default: "UK" },
      postcode:String
    },
  }
});

const personModel = model("collection", personSchema);
const document = new personModel({
  info: {
    address: { country: "United States", city: "New York" },
  }
});
console.log(document);

document.$set({ info: { address: { postcode: "12H" } } }, null, { merge: true });

console.log(document);

Expected Behavior

For this code example I get

{
  info: { address: { postcode: '12H', country: 'UK' } },
  _id: new ObjectId('66d693ee34802938adef5702')
}

I would expect to get

{
  info: { address: { city: 'New York', country: 'United States',  postcode: '12H' } },
  _id: new ObjectId('66d692b9c9fe017af8ab4f78')
}

[ianHeydoc]

I've taken a look into the code and found this code snippet

this is on line 1212 of mongoose/lib/document.js

      if (!merge) {
        this.$__setValue(path, null);
        cleanModifiedSubpaths(this, path);
      } else {
        return this.$set(val, path, constructing);
      }

It looks like the merge option is lost when the second nested object is set

I've tested out adding the options to the next $set call and it looks like it works 👀

Would this be something I could open a pr to implement or is there a reason for the behaviour?

I'm new to looking at the source code for mongoose so any help would be appreciated 😃

[pdrf]

+1

[vkarpov15]

Should be fixed by #14870