I've been unable to create MongoDb documents nested to a third level, using the NET driver for MongoDb. The top level class has a List of the second level objects. The second level class has a list of third level objects. However, the insert operation fails to include the third level doc data. The project is a console application using MongDB.Driver v3.7.

I also tried to manually register the third level Doc definition, but that code may be incorrect. However, is it necessary to register all documents, below the second level for the driver, to "detect" it?

Class definitions for documents:

public class LibraryUser
{
    [BsonRepresentation(BsonType.ObjectId)]
    public string Id { get; set; } = ObjectId.GenerateNewId().ToString();
    public required int UserId { get; set; }
    public List Books { get; set; } = [];
    public void AddBook(Book book)
    {
        Books.Add(book);
    }
}

public class Book
{
    [BsonRepresentation(BsonType.ObjectId)]
    public string Id { get; set; } = ObjectId.GenerateNewId().ToString();
    public string BookTitle { get; set; }
    public int Price { get; set; }
    List Reviewers { get; set; } = [];
    public void addReview(Review condition)
    {
        Reviewers.Add(condition);
    }
}

public class Review
{
    [BsonRepresentation(BsonType.ObjectId)]
    public string Id { get; set; } = ObjectId.GenerateNewId().ToString();
    public string Reviewer { get; set; }
    public int Grade { get; set; }
}

Code to create a nested document:

iMongoDatabase.CreateCollection("LibraryUsers");
LibraryUsersCollection = iMongoDatabase!.GetCollection("LibraryUsers");
LibraryUser userDoc = new()  // First Level 
{
    UserId = 999999
};
Book book = new()  // Second Level 
{
    BookTitle = "Old Yeller",
    Price = 11
};
Review review = new() // Third Level 
{
    Reviewer = "Corin",
    Grade = 55
};
book.addReview(review); // Add Review doc to Book 
userDoc.AddBook(book);  // Add Book doc to LibraryUser doc
LibraryUsersCollection.InsertOne(userDoc);

Code for registering Review class - which did not help

MongoDB.Bson.Serialization.BsonClassMap.RegisterClassMap(classmap =>
{
    classmap.AutoMap();
    classmap.MapMember(p => p.Reviewer);
    classmap.MapMember(p => p.Grade);
});

Resultant data viewed with Mongosh:

db.LibraryUsers.find()
[
  {
    _id: ObjectId('69e22dcac06b765666fcda19'),
    UserId: 999999,
    Books: [
      {
        _id: ObjectId('69e22dcac06b765666fcda1a'),
        BookTitle: 'Old Yeller',
        Price: 11
      }
    ]
  }
]

GitHub repo