Mongoose 8 type: Buffer can set but will not be returned

[rwmia]

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.x.x

Node.js version

22.x

MongoDB server version

5.x

Typescript version (if applicable)

No response

Description

Mongoose 8 seems to have an issue with type: Buffer field. It will set a buffer, but not return one. Will only work if the type is Any.

Mongoose will save type:Buffer as normal, but when retrieving the same document it will not return the type: Buffer. If you instead set field to type: Any, then it returns fine. Mongoose 6 worked as intended, however in Mongoose 7+ it seems to have stopped returning a buffer field.

In v8 when field is type: Buffer, saves as Binary.createFromBase64('xxx...), but will not return the field when retrieving the document.

When changing the field schema to type: Any, it "works" but saves as an object {"type":"buffer", "data": [...]}

Steps to Reproduce

Create a document with a schema field of type: Buffer.
Update the document field with a buffer.
Retrieve the document and the field will be undefined.

Instead of type: Buffer, use type: Any, or just {};
Update the document field with a buffer.
Retrieve the document and the field is returned as an object.

Expected Behavior

Should work as in Mongoose 6, saves type: Buffer and will retrieve it as well.

[vkarpov15]

I'm unable to repro, the following script shows that bufferField is a buffer both after create() and findOne(). Please modify the following script to demonstrate the issue you're seeing.

const mongoose = require('mongoose');

const MONGO_URI = 'mongodb://127.0.0.1:27017/mongoose_test';

const testSchema = new mongoose.Schema({
  bufferField: { type: Buffer },
});
const TestModel = mongoose.model('Test', testSchema);

run().catch(err => console.log(err));

async function run() {
  await mongoose.connect(MONGO_URI);
  await TestModel.deleteMany({});
  
  // Insert document with Buffer type
  const bufferData = Buffer.from('Hello, Mongoose 8!');
  const testDoc1 = await TestModel.create({ bufferField: bufferData });

  console.log(Buffer.isBuffer(testDoc1.bufferField));
  testDoc1.bufferField = Buffer.from('Hello!');
  await testDoc1.save();
  
  const { bufferField } = await TestModel.findById(testDoc1._id).orFail();
  console.log(Buffer.isBuffer(bufferField));

  process.exit(0);
}