Auto population of virtual fields with reference to other models

[Freezystem]

Hi,
I have a question regarding auto population of virtual fields.
The behavior I get is not what I expect from the auto virtual population when using {toJSON: {virtuals: true}} in the schema definition.

Here is a test I'm expecting to pass.
I'm expecting the virtual property author.books to be automatically populated with the list of ids until I explicitely use populate("books") to populate with expanded document.

describe("Model Virtual", () => {
  it("Should auto populate id list", async () => {
    const Author = mongoose.model(
      "Author",
      new mongoose.Schema(
        { name: String },
        {
          virtuals: {
            books: {
              options: {
                ref: "Book",
                localField: "_id",
                foreignField: "author",
            },
        },
      },
      toJSON: { virtuals: true },
      toObject: { virtuals: true },
    }));

    const Book = mongoose.model(
      "Book",
      new mongoose.Schema({
        name: String,
        author: mongoose.Schema.Types.ObjectId,
    }));

    const author = new Author({ name: "author" });
    await author.save();
    const book1 = new Book({ name: "book1", author: author._id });
    await book1.save();
    const book2 = new Book({ name: "book2", author: author._id });
    await book2.save();

    const _author = await Author.findById(author._id).exec();
    const authorJSON = _author?.toJSON();
    expect(authorJSON).toHaveProperty("books", [book1._id, book2._id]); // fails as books is undefined
    
    await _author?.populate("books");
    expect(_author?.books?.map(b => b.name)).toBe(["book1", "book2"]); // populates books as expected
  });
});

I'm wondering if I do something wrong or if I don't understand the documentation properly.