Populate with select on nested field returns extra sub-field of array type.

[mattinbits]

Prerequisites

• I have written a descriptive issue title
• I have searched existing issues to ensure the bug has not already been reported

Mongoose version

6.9.0

Node.js version

16.13.1

MongoDB server version

6.0.3

Typescript version (if applicable)

4.1.3

Description

A parent schema contains a reference to a child schema. The child schema contains a nested field, with a string property and an array property. When populating a parent and selecting the nested string property, the array property is also included, with empty contents.

Steps to Reproduce

Example code:

import mongoose from "mongoose";

export const ParentSchema = new mongoose.Schema({
  title: String,
  childId: { type: mongoose.Schema.Types.ObjectId, ref: "Child" }
});

export const Parent = mongoose.model("Parent", ParentSchema);

const childSchema = new mongoose.Schema({
  name: String,
  properties: { foo: String, bar: [{ baz: String, qux: Boolean }] }
});

export const Child = mongoose.model("Child", childSchema);

async function run() {
  const child = await Child.create({ name: "c1", properties: { foo: "foo1", bar: [] } });

  const parent = await Parent.create({ title: "p1", childId: child._id });

  const res = await Parent.findOne({ title: "p1" }).populate({
    path: "childId",
    model: Child,
    select: { "properties.foo": 1 }
  });
  return res;
}

mongoose
  .connect("mongodb://localhost:27017/test")
  .then((_) => run())
  .then((res) => console.log(JSON.stringify(res)));

This prints:

{
  "_id": "63e16b1747e00097cf9a5957",
  "title": "p1",
  "childId": {
    "properties": { "foo": "foo1", "bar": [] },
    "_id": "63e16b1747e00097cf9a5955"
  },
  "__v": 0
}

Note that the bar field is included, unexpectedly. If I populate the bar array in the child document, such as:

const child = await Child.create(
    {
        name: "c1", 
        properties: { foo: "foo1", bar: [{baz: "quk", qux: true}] } 
    });

Then I still get an empty bar array in the populated parent document.

I don't encounter the same problem when bar is a simple string property instead of an array of objects, e.g.:

properties: { foo: String, bar: String }

Using the original example with Mongoose 5.11.5 I get the expected result:

{
  "_id": "63e16b1747e00097cf9a5957",
  "title": "p1",
  "childId": {
    "properties": { "foo": "foo1" },
    "_id": "63e16b1747e00097cf9a5955"
  },
  "__v": 0
}

Expected Behavior

The populated Parent document contains only the properties.foo field, similar to with mongoose 5.x.

[IslandRhythms]

const mongoose = require('mongoose');

const ParentSchema = new mongoose.Schema({
  title: String,
  childId: { type: mongoose.Schema.Types.ObjectId, ref: "Child" }
});

const Parent = mongoose.model("Parent", ParentSchema);

const childSchema = new mongoose.Schema({
  name: String,
  properties: { foo: String, bar: [{ baz: String, qux: Boolean }] }
});

const Child = mongoose.model("Child", childSchema);

async function run() {
  await mongoose.connect('mongodb://localhost:27017');
  await mongoose.connection.dropDatabase();
  const child = await Child.create({ name: "c1", properties: { foo: "foo1", bar: [] } });

  const parent = await Parent.create({ title: "p1", childId: child._id });

  const res = await Parent.findOne({ title: "p1" }).populate({
    path: "childId",
    model: Child,
    select: { "properties.foo": 1 }
  });
  console.log(res);
}

run();