[Bug] Version mode can be downgraded when modifying multiple paths

[AbdelrahmanHafez]

Prerequisites

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

Mongoose version

9.0.

Node.js version

N/A

MongoDB server version

N/A

Typescript version (if applicable)

N/A

Description

When modifying multiple paths in a single save, the version mode (this.$__.version) can be incorrectly downgraded. This happens because the versioning logic uses direct assignment instead of accumulating with bitwise OR.

For example, if you:

1. Push to a top-level array (should set VERSION_INC = 2)
2. Set a value on an array index path (should set VERSION_WHERE = 1)

The second operation overwrites the first, resulting in VERSION_WHERE (1) instead of VERSION_ALL (3). This means __v is added to the query filter but NOT incremented, even though we pushed to an array.

Steps to Reproduce

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

run();

async function run() {
  await mongoose.connect('mongodb://127.0.0.1:27017/test');

  const Model = mongoose.model('Test', new mongoose.Schema({
    items: [{ name: String }],
    tags: [String]
  }));

  const doc = await Model.create({
    items: [{ name: 'item1' }, { name: 'item2' }, { name: 'item3' }],
    tags: ['tag']
  });

  // Two clients load the same document
  const clientA = await Model.findById(doc._id);
  const clientB = await Model.findById(doc._id);

  // Client A modifies the document
  clientA.items.pull(clientA.items[0]._id); // pull (VERSION_INC)
  clientA.tags[0] = 'modified'; // set index (VERSION_WHERE overwrites it)
  await clientA.save();
  // items is now ['item2', 'item3'], __v should be 1

  const afterA = await Model.findById(doc._id);
  assert.strictEqual(afterA.__v, 1, '__v should be incremented after pull');

  // Client B has stale view, thinks items[1] is "item2"
  clientB.items[1].name = 'item2-updated';
  const err = await clientB.save().then(() => null, err => err);
  assert.ok(err, 'save should throw VersionError due to stale __v');
  assert.strictEqual(err.name, 'VersionError');
}

Expected Behavior

When modifying multiple paths that trigger different version modes, the modes should be combined using bitwise OR:

• VERSION_WHERE (1) + VERSION_INC (2) = VERSION_ALL (3)

This ensures that:

1. __v is included in the query filter (for index safety)
2. __v is incremented (to signal array position changes to other clients)