Wrong document updating after pulling embedded object with _id field alias

[Kamikadze4GAME]

Do you want to request a feature or report a bug?
Bug

What is the current behavior?
Inconsistent object saving after .pull(id) if you disable _id in the scheme of the subfield (MongooseArray) and use another field that has an alias _id.

new Schema({
 children: [
   new Schema({
     field: {
       type: String,
       alias: '_id'
     }
   }, { _id: false })
 ]
});

If the current behavior is a bug, please provide the steps to reproduce.

'use strict';

const mongoose = require('mongoose');
mongoose.set('debug', true);

const GITHUB_ISSUE = `gh9319`;
const connectionString = `mongodb://localhost:27017/${GITHUB_ISSUE}`;
const { Schema } = mongoose;

run()
  .then(() => console.log('done'))
  .catch(error => console.error(error.stack))
  .then(() => process.exit(0));

async function run() {
  await mongoose.connect(connectionString, { useNewUrlParser: true, useUnifiedTopology: true });

  await mongoose.connection.dropDatabase();

  const childSchema = new Schema({
    field: {
      type: String,
      alias: '_id'        // <--------┐
    }                     //          ├------ important
  }, {                    //          |
    _id: false            // <--------┘
  });

  const parentSchema = new Schema({
    children: [childSchema]
  });

  const Parent = mongoose.model('Parent', parentSchema);


  // Step 1. Create parent with children and find it.
  await Parent.create({
    children: [
      { field: '1' }
    ]
  });
  // Mongoose: parents.insertOne({ _id: ObjectId("5f3395ecd2702ec3f4f70fc4"), children: [ { field: '1' } ], __v: 0}, { session: null })

  // Step 2. Find the parent just created
  const parent = await Parent.findOne();
  // Mongoose: parents.findOne({}, { projection: {} })

  console.log(parent);  // { _id: 5f3395ecd2702ec3f4f70fc4, children: [ { field: '1' } ], __v: 0 }

  // It's possible to find child by id
  let child = parent.children.id('1');

  console.log(child); // { field: '1' }


  // Step 3. Pull (remove) child by id
  parent.children.pull('1');
  // There will be same final result (and debug log `Mongoose: parents.updateOne(...)`) if use one of the following:
  //   parent.children.pull({_id: '1'})
  // or
  //   parent.children.pull({field: '1'})
  //
  // But
    // child.remove(); // throws new Error('For your own good, Mongoose does not know how to remove an EmbeddedDocument that has no _id')



  // Now `children` is empty array
  console.log(parent); // { _id: 5f3395ecd2702ec3f4f70fc4, children: [], __v: 0 }

  // And `child` is null
  child = parent.children.id('1');

  console.log(child); // null


  // Step 4. Save parent and find it again
  const parent_1 = await parent.save();
  // Mongoose: parents.updateOne({ _id: ObjectId("5f3395ecd2702ec3f4f70fc4") }, { '$pull': { children: { _id: { '$in': [ '1' ] } } }, '$inc': { __v: 1 }}, { session: undefined })
  //                                                                                                      ^
  //                                                                                                  Look here

  // `parent_1` equals `parent`
  console.log(parent_1); // { _id: 5f3395ecd2702ec3f4f70fc4, children: [], __v: 1 }


  const parent_2 = await Parent.findOne();
  // Mongoose: parents.findOne({}, { projection: {} })

  // Theoretically `parent_1` and `parent_2` should be equal, but they are NOT.
  console.log(parent_2); // { _id: 5f3395ecd2702ec3f4f70fc4, children: [ { field: '1' } ], __v: 1 }


  await mongoose.disconnect();
}

There problem is here.

What is the expected behavior?
Objects parent, parent_1 and parent_2 should be equal.

Possible solutions:

Solution 0 (just hotfix)
Change this line

        return v._id || v;

with

        return v._doc._id || v;

Solution 1 (bad)
Make .pull to mark all array as modified like .shift(), .pop() etc.

Solution 2 (not so bad)
Make .pull to mark all array as modified if last element was removed.

Solution 3 (better)
Make .pull to understand that pathType of _id is not real but virtual (or maybe new one alias) and should use smth like Model.translateAliases()

Solution 4 (also not bad)
Allow to set option _id with field alias for Subschemas. Example:

const childSchema = new Schema({
  field: {
    type: String,
  }
}, {
  _id: 'field'
});

What are the versions of Node.js, Mongoose and MongoDB you are using? Note that "latest" is not a version.

Node.js: v14.8.0
Mongoose: v5.9.28
MongoDB: v4.4.0

[Kamikadze4GAME]

@vkarpov15 sorry for mention, but did you check this bug?

[vkarpov15]

Thanks for the detailed repro and investigation. We're going to go with solution 0, I think that's more consistent with our existing behavior. We currently don't automatically call translateAliases() internally, so I think we should treat parent.children.pull('1') as parent.children.pull({ field: 1 }) in this case, because we still end up casting '1' to { field : 1 }.

[Kamikadze4GAME]

@vkarpov15 Thank you!