.cursor({ transform: JSON.stringify }) doesn't work the same way for AggregationCursor as in QueryCursor (when piped to response)

[kemalcany]

Prerequisites

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

Mongoose version

8.1.1

Node.js version

18.13.0

MongoDB server version

6.0.4

Typescript version (if applicable)

5.3.3

Description

The { transform: JSON.stringify } option for a cursor doesn't produce the expected result on aggregation cursors while they work fine with query cursors.

The example below with .find() works well and you can get the streamed documents on the client

@Get('apples')
async getApplesWithQuery(@Response() res) {
   Fruit.find({name:"Apple"}).cursor({ transform: JSON.stringify }).pipe(res.type('json'))
}

Whereas the same request (below) with aggregation/$match doesn't work and you get an error like the one at the bottom:

@Get('apples')
async getApplesWithAggregation(@Response() res) {
   Fruit.aggregate([{$match:{name:"Apple"})}]).cursor({ transform: JSON.stringify }).pipe(res.type('json'))
}

TypeError: The "chunk" argument must be of type string or an instance of Buffer or Uint8Array. Received an instance of Object
    at new NodeError (node:internal/errors:400:5)
    at write_ (node:_http_outgoing:862:11)
    at ServerResponse.write (node:_http_outgoing:827:15)
    at AggregationCursor.ondata (node:internal/streams/readable:766:22)
    at AggregationCursor.emit (node:events:513:28)
    at addChunk (node:internal/streams/readable:324:12)
    at readableAddChunk (node:internal/streams/readable:297:9)
    at AggregationCursor.Readable.push (node:internal/streams/readable:234:10)
    at /XXX/node_modules/.pnpm/mongoose@8.1.1/node_modules/mongoose/lib/cursor/aggregationCursor.js:96:11
    at /XXX/node_modules/.pnpm/mongoose@8.1.1/node_modules/mongoose/lib/cursor/aggregationCursor.js:375:

Steps to Reproduce

Replacing a find query with $match aggregation on a cursor response can demonstrate the issue.

Note that the aggregation cursor works well. ie if you do simply read the cursor with .next(), you can get the values fine. The issue happens when you try to pipe it as a response due to the fact that the results are not getting JSON.stringified as they would on a query cursor

Expected Behavior

No response

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[IslandRhythms]

const mongoose = require('mongoose');
const { Writable } = require('stream')
const testSchema = new mongoose.Schema({
  name: String
});

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

async function run() {
  await mongoose.connect('mongodb://localhost:27017');
  await mongoose.connection.dropDatabase();

  for(let i = 0; i < 11; i++) {
    await Test.create({ name: 'Apple' });
  }
  for(let i = 0; i < 11; i++) {
    await Test.create({ name: 'Orange'})
  }

  const writableStream = new Writable({
    write(chunk, encoding, callback) {
      console.log(chunk.toString()); // Log the chunk to the console
      callback(); // Call the callback to indicate that processing is complete
    }
  });

  const otherStream = new Writable({
    write(chunk, encoding, callback) {
      console.log(chunk.toString()); // Log the chunk to the console
      callback(); // Call the callback to indicate that processing is complete
    }
  });

  console.log('find');
  await Test.find({ name: 'Apple' }).cursor({ transform: JSON.stringify }).pipe(writableStream);
  console.log('aggregate');
  await Test.aggregate([{$match: { name: 'Orange' }}]).cursor({ transform: JSON.stringify }).pipe(otherStream);

  console.log('done');
}

run();