Discriminators: Return derived model when issuing Find on base model

[TomMettam]

Prerequisites

• I have written a descriptive issue title

Mongoose version

8.3.4

Node.js version

v20.11.1

MongoDB version

5.0.16-r1::gentoo

Operating system

Linux

Operating system version (i.e. 20.04, 11.3, 10)

Gentoo

Issue

I have just finished working on updating an old codebase.

As part of this work, I migrated from the old and largely defunct mongoose-extend-schema to Mongoose Discriminators.

However, there's a key change in functionality which is causing a significant loss in performance.

const baseModelSchema = new mongoose.Schema({
    ...
});

const derivedModelSchema = new mongoose.Schema({
    ...
});

const baseModel = connection.model('BaseModel', baseModelSchema);
const derivedModel = baseModel.discriminator('DerivedModel', derivedModelSchema);

With mongoose-extend-schema, if I did:

const result = await baseModel.findOne({ ... });

and the discriminator matched derivedModel, it would give me an instance of derivedModel.

With discriminators, it always gives me an instance of baseModel which means that I have to manually convert it, which costs a bunch of additional CPU cycles...

baseModelSchema.post('find', function (docs, next)
    {
        if (docs?.length)
        {
            for (let index = 0; index < docs.length; index++)
            {
                const doc = docs[index];
                const Model = connection.model(doc.__type);
                docs[index] = new Model(doc.toObject());
            }
        }
        next();
    });

This is suboptimal and slow, and I'd really like it if mongoose could give me the correct instance for the model. Is there an option to enable this?

[github-actions]

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[vkarpov15]

I'm unable to repro, AFAIK Mongoose does give you the correct discriminator model with findOne(). For example, consider the following script. Can you please modify the following repro script to demonstrate the issue you're seeing?

'use strict';

const mongoose = require('mongoose');

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

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

  const baseModelSchema = new mongoose.Schema({ baseModelProp: String });

  const derivedModelSchema = new mongoose.Schema({ derivedModelProp: String });

  const baseModel = mongoose.model('BaseModel', baseModelSchema);
  const derivedModel = baseModel.discriminator('DerivedModel', derivedModelSchema);

  await baseModel.deleteMany({});
  const { _id } = await derivedModel.create({ baseModelProp: 'base model test', derivedModelProp: 'derived model test' });

  const doc = await baseModel.findById(_id).orFail();
  console.log(doc.derivedModelProp); // Prints "derived model test"
  console.log(doc instanceof derivedModel); // Prints "true"
}