Populating a field of the items in an array, adds the field to every item regardless of its schema

[rstcruzo]

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

What is the current behavior?
Populating an array with multiple discriminators adds the field as null or empty array even when the item schema doesn't have the field defined. This happens when the populated field is either an array or a virtual.

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

const mongoose = require('mongoose');
const {Schema, ObjectId, model} = mongoose;

const ImageSchema = new mongoose.Schema({
    imageName: {
        type: String,
        required: true
    }
}, {_id: false});

const TextSchema = new mongoose.Schema({
    textName: {
        type: String,
        required: true
    },
    attached: [{
        type: mongoose.Schema.Types.ObjectId,
        ref: 'AnotherModel'
    }]
}, {_id: false});

const ItemSchema = new Schema({
    objectType: {
        type: String,
    },
}, {
    discriminatorKey: 'objectType',
    _id: false,
});

const ExampleSchema = new Schema({
    test: {
        type: String,
    },
    list: [{
        type: ItemSchema,
        required: false
    }],
});

ExampleSchema.path('list').discriminator('Image', ImageSchema);
ExampleSchema.path('list').discriminator('Text', TextSchema);
const Example = model('Example', ExampleSchema);

const example = await Example.create({
    test: 'example',
    list: [
        {
            imageName: "an image",
            objectType: 'Image',
        },
        {
            textName: "this is a text",
            attached: ["61254490ea89de0004c8f2a0"],
            objectType: 'Text',
        },
    ],
});
const query = Example
    .findById(example._id)
    .populate('list.attached')
    .lean()

const result = await query.exec();

return result;

What is the expected behavior?
result should be:

const result = {
    test: 'example',
    list: [
        {
            imageName: "an image",
            objectType: 'Image',
        },
        {
            textName: "this is a text",
            attached: [{field: 'value'}],
            objectType: 'Text',
        },
    ],
}

result is:

const result = {
    test: 'example',
    list: [
        {
            attached: [],  // extra field. Image doesn't have a field called attached.
            imageName: "an image",
            objectType: 'Image',
        },
        {
            textName: "this is a text",
            attached: [{field: 'value'}],
            objectType: 'Text',
        },
    ],
}

What are the versions of Node.js, Mongoose and MongoDB you are using? Note that "latest" is not a version.
Nodejs: 12.21.0
mongoose: 5.12.1
mongodb: 3.2.22

[IslandRhythms]

@rstcruzo could you please include what AnotherModel is? I'm reproing it right now and want to make sure that AnotherModel doesn't affect the output.

[IslandRhythms]

this is result {
  _id: new ObjectId("61295305a763d02001b053f4"),
  test: 'example',
  list: [
    { imageName: 'an image', objectType: 'Image', attached: [] },
    { textName: 'this is a text', attached: [], objectType: 'Text' }
  ],
  __v: 0
}

I'm not getting the same thing you are. I don't know if that has to do with schema for AnotherModel

[rstcruzo]

@IslandRhythms thanks for looking into this. That's exactly what I get. The image item now contains the attached field even though its schema doesn't have it.

[rstcruzo]

@vkarpov15 Thanks! That fixed it for regular fields, not for virtuals though. Should we reopen the issue?

[vkarpov15]

@rstcruzo can you please clarify what you mean by "not for virtuals"?