5.12.6 dropped support for 4.4 projection expression

critical bug

What is the current behavior?

mongoose version 5.12.6 dropped support for MongoDB 4.4 projection expressions. In version 5.12.5 everything works as expected.

Reproduce script:

const mongoose = require('mongoose');
const Schema = mongoose.Schema;

const dbConnection = require('./conn')

const TestSchema = new Schema({
    field: {
        subField: String
    }
});

TestSchema.statics.getSubField = function () {
    return this.findOne({}, {
        subField: '$field.subField'
    }).lean().exec();
}

async function test () {
    const connection = await dbConnection();
    const TestModel = connection.model('TestModel', TestSchema);
    const newTestModel = new TestModel({field: {subField: 'schema sub field value'}});
    await newTestModel.save();

    const res = await TestModel.getSubField();
    console.log(res.subField);

    await TestModel.deleteMany({}).exec();
}

test();

With version 5.12.5 console output:

/usr/bin/node /home/tom/WebstormProjects/test/model.js schema sub field value

With version 5.12.6 console output:

/usr/bin/node /home/tom/WebstormProjects/test/model.js undefined

same happens with find

What is the expected behavior? the projection in find and findOne queries, should support aggregation projection expressions, as in 5.12.5 and MongoDB 4.4

What are the versions of Node.js, Mongoose and MongoDB you are using? Note that “latest” is not a version. Node.js - 14.16.1 Mongoose - 5.12.6 MongoDB - 4.4

[vkarpov15]

I took a closer look and you’re right @tomgrossman , I’m very sorry for the difficulty. We made this fix in #10142 as a precaution, but you’re right that it also hides a useful feature. We will undo this change and release the fix in v5.12.9 👍

[IslandRhythms]

@tomgrossman