Inconstancy with .set("key", undefined) in middlewares

[Jule-]

Prerequisites

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

Mongoose version

6.4.1

Node.js version

16.14.2

MongoDB server version

4.2.3

Description

Depending on how you define your update payload, it will result in different behaviours in Query middlewares when you try to set some property to undefined with this.set("key", undefined) method.

1. without $set: setting to undefined have no effect and will use property value from update object
2. with $set: setting to undefined will effectively remove property value from update object

NB: I have not checked for Document middlewares but it could be the same.

Steps to Reproduce

const testSchema = new Schema({
  key: { type: String, unique: true, sparse: true },
  // [Use Case] `unique` and `sparse` not necessary just here because it
  // illustrate a real case scenario impacted by this issue.
});

testSchema.pre('findOneAndUpdate', function (next) {
  if (this.get('key') === '') this.set('key', undefined);
  return next();
});

const Test = mongoose.model('Test', testSchema);

// [Use Case] Getting empty string from web client and you do not want to
// rely on usage to clean the update object and ensure what is/should be
// bound to the scheme definition

await Test.findOneAndUpdate({}, { key: '' });
// => { $set: { apiKey: '' } }

await Test.findOneAndUpdate({}, { $set: { key: '' } });
// => { $set: {} }

Expected Behavior

I would expect that both usage of query.set('key', undefined) results in the same generated query update { $set: {} }.

[Jule-]

mongoose/lib/query.js

Lines 2031 to 2044 in 62f5c33

	Query.prototype.set = function(path, val) {
	if (typeof path === 'object') {
	const keys = Object.keys(path);
	for (const key of keys) {
	this.set(key, path[key]);
	}
	return this;
	}
	this._update = this._update || {};
	this._update.$set = this._update.$set || {};
	this._update.$set[path] = val;
	return this;
	};

Here we can see that regarding the failing use case we have this._update equals to { key: "" } and set will change it to {key: "", $set: { key: undefined }}.

Then I guess that the query resolution will default top-level field assignments into $set which loses the information that the middleware wants to set key to undefined.

[IslandRhythms]

const mongoose = require('mongoose');
mongoose.set('debug', true);

const testSchema = new mongoose.Schema({
    key: { type: String, unique: true, sparse: true },
    // [Use Case] `unique` and `sparse` not necessary just here because it
    // illustrate a real case scenario impacted by this issue.
  });
  
  testSchema.pre('findOneAndUpdate', function (next) {
    if (this.get('key') === '') this.set('key', undefined);
    return next();
  });
  
  const Test = mongoose.model('Test', testSchema);
  
  // [Use Case] Getting empty string from web client and you do not want to
  // rely on usage to clean the update object and ensure what is/should be
  // bound to the scheme definition
  

  async function run() {
    await mongoose.connect('mongodb://localhost:27017');
    await mongoose.connection.dropDatabase();
    await Test.findOneAndUpdate({}, { key: '' });
    // => { $set: { apiKey: '' } }
    
    await Test.findOneAndUpdate({}, { $set: { key: '' } });
    // => { $set: {} }
  }

  run();