Error: TypeError: Cannot read properties of undefined (reading '_id')
    at _assign (//node_modules/mongoose/lib/model.js:4471:38)
    at _done (/node_modules/mongoose/lib/model.js:4300:9)
    at _next (//node_modules/mongoose/lib/model.js:4288:7)
    at //node_modules/mongoose/lib/model.js:4396:7
    at process.processTicksAndRejections (node:internal/process/task_queues:95:5)

import mongoose from "mongoose";

await mongoose.connect("mongodb://localhost:27017/blogdb");

const authorSchema = new mongoose.Schema({
  group: String, // our populate key
  name: String,
});

const postSchema = new mongoose.Schema({
  authorGroup: String,
  title: String,
  content: String,
});

const Author = mongoose.model("Author", authorSchema);
const Post = mongoose.model("Post", postSchema);

async function createTestData() {
  await Author.create({ group: "AUTH1", name: "John Doe" });
  await Author.create({ group: "AUTH2", name: "Jane Smith" });
  await Author.create({ group: "AUTH2", name: "Will Jons" }); // second author with the same group (it will cause an issue). One populated record works just fine

  await Post.create({
    authorGroup: "AUTH1",
    title: "First Post",
    content: "Content 1",
  });
  await Post.create({
    authorGroup: "AUTH2",
    title: "Second Post",
    content: "Content 2",
  });
}

async function runProblemQuery() {
  try {
    const posts = await Post.find()
      .populate({
        path: "authorGroup",
        model: "Author",
        select: { _id: 1, name: 1 },
        foreignField: "group",
      })
      .select({ _id: 1, authorGroup: 1, title: 1 })
      .lean();

    /**
     * WARNING: Bug detected in Mongoose 8.5.2 when using .lean() with .populate()
     *
     * Issue: When using .lean() in combination with .populate() and a custom foreignField,
     * Mongoose fails to correctly populate the related documents.
     */
    console.log(
      "postPopulate>>>>>>",
      JSON.stringify(posts, null, 2)
    );
  } catch (error) {
    console.error("Error:", error);
  }
}

async function runTest() {
  await createTestData();
  await runProblemQuery();
  mongoose.connection.close();
}

runTest();