Strange behavior with Transactions and document updates about mongoose HOT 2 OPEN

A few more updates: after running some more tests it seems like this issue may be related to Document state not resetting between transaction retries.

If we modify the the second operation to force a field to be 'modified', everything works as expected:

transaction_builder.addOperation(async (session) => {
   console.log('updating in banking config');
   company.markModified('features.banking_config');
   await company.save({ session });
 });

However, my assumption is that this will only apply updates to that specific field; to ensure that all updates are applied properly, we would need to diff the og object with the updated object and mark each updated field as 'modified' for each subsequent transaction retry.

So, a couple questions:

1. Why does the document state not fully revert when a transaction aborts and 'rolls back'?
2. What's the best way to ensure that the document remembers what fields have been modified between transaction retries?
3. How can a call to document.save() resolve to a findOne() operation? Is this documented anywhere?

Thank you kindly,
Taylor

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I managed to create a more simplified repro. The issue is that Mongoose's document state doesn't revert when the transaction aborts, and save() becomes a findOne() if there's no changes to save.

const mongoose = require('mongoose');

mongoose.set('debug', true);

main().catch(error => {
  console.error(error);
  process.exit(-1);
});

async function main() {
  await mongoose.connect('mongodb://127.0.0.1:27017,127.0.0.1:27018/mongoose_test?replicaSet=rs');

  const Test = mongoose.model('Test', mongoose.Schema({ name: String }));
  await Test.deleteMany({});
  const { _id } = await Test.create({ name: 'foo' });

  const doc = await Test.findById(_id);
  doc.name = 'bar';
  for (let i = 0; i < 2; ++i) {
    const session = await mongoose.startSession();
    session.startTransaction();

    await doc.save({ session });
    if (i === 0) {
      await session.abortTransaction();
    } else {
      await session.commitTransaction();
    }
    await session.endSession();
  }

  console.log(await Test.findById(_id)); // Still prints name: 'foo'
}

The easiest workaround is to use Mongoose's transaction() helper, which tracks and resets document state if the transaction errors.

Another workaround is to avoid modifying documents that are external to the transaction. For example, the following works fine:

  for (let i = 0; i < 2; ++i) {
    const session = await mongoose.startSession();
    session.startTransaction();

    const doc = await Test.findById(_id);
    doc.name = 'bar';

    await doc.save({ session });
    if (i === 0) {
      await session.abortTransaction();
    } else {
      await session.commitTransaction();
    }
    await session.endSession();
  }

Do either of these approaches work for you?

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