Re-assigning a DocumentArray with self elements in different order does not change `__index`

[hasezoey]

Prerequisites

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

Mongoose version

9.1.4

Node.js version

All

MongoDB server version

Any

Typescript version (if applicable)

5.9.3 (not the issue)

Description

When re-assigning a DocumentArray with a plain array, but have the elements be in a different order, does not change their internal modified indexes.

Original Issue: typegoose/typegoose#1025

Steps to Reproduce

// NodeJS: 25.2.1
// MongoDB: 7.0 (Docker)
// Typescript 5.9.3
import * as mongoose from 'mongoose'; // mongoose@9.1.4

const child = new mongoose.Schema({
  id: Number,
  v: Number,
});

const parent = new mongoose.Schema({
  _id: String,
  array: {
    type: [child],
    _id: false,
  },
});

const TestModel = mongoose.model('test', parent);

async function main() {
  await mongoose.connect(`mongodb://localhost:27017/`, {
    dbName: 'verifyMASTER',
  });

  await TestModel.deleteOne({ _id: 'test' });
  await TestModel.create({
    _id: 'test',
    array: [
      { id: 1, v: 0 },
      { id: 2, v: 0 },
    ],
  });

  const doc = await TestModel.findById('test').orFail();

  // switch positions
  // @ts-expect-error Type mismatch, assigning a plain array to something that is "DocumentArray"
  doc.array = [doc.array[1], doc.array[0]];
  // [doc.array[1], doc.array[0]] = [doc.array[0], doc.array[1]]; // this one actually works
  await doc.save(); // [ { "id" : 2, "v" : 0 }, { "id" : 1, "v" : 0 } ]
  const fetched1 = await TestModel.findById(doc).orFail().lean();
  // first true { _id: 'test', array: [ { id: 2, v: 0 }, { id: 1, v: 0 } ], __v: 1 }
  console.log('first', JSON.stringify(doc) === JSON.stringify(fetched1));
  console.log('doc', doc);
  console.log('fetched1', fetched1);

  doc.array[0].v = 999;
  // modified [ 'array', 'array.1', 'array.1.v' ]
  // ^ should be "array.0"
  console.log('modified', doc.modifiedPaths());
  await doc.save(); // nothing changed [ { "id" : 2, "v" : 0 }, { "id" : 1, "v" : 0 } ]
  const fetched2 = await TestModel.findById(doc).orFail().lean();
  // second false { _id: 'test', array: [ { id: 2, v: 999 }, { id: 1, v: 0 } ], __v: 1 }
  console.log('second', JSON.stringify(doc) === JSON.stringify(fetched2));
  // { _id: 'test', array: [ { id: 2, v: 999 }, { id: 1, v: 0 } ], __v: 1 }
  console.log('doc', doc);
  // { _id: 'test', array: [ { id: 2, v: 0 }, { id: 1, v: 0 } ], __v: 1 }
  console.log('fetched2', fetched2);

  await mongoose.disconnect();
}

main();

Reproduction Repository & Commit

Expected Behavior

To have the swapped element's __index element updated to their respective new position.

[dsjstapler]

Changing doc.array = [doc.array[1], doc.array[0]] to doc.array = new mongoose.Types.DocumentArray([doc.array[1], doc.array[0]]) results in no TypeScript error and exhibits the same issue, which may help illustrate the problem more clearly.