one-to-many entity relationship example

Program.cs

using MongoDB.Driver;
using MongoDB.Driver.Linq;
using MongoDB.Entities;
using System.Linq;
using System.Threading.Tasks;

namespace ReferencedRelationshipExample
{
    public class Author : Entity
    {
        public string Name { get; set; }
        public Many<Book> Books { get; set; }

        public Author() => this.InitOneToMany(() => Books);
    }

    public class Book : Entity
    {
        public string Title { get; set; }
        public One<Author> Author { get; set; }
    }

    public class AuthorWithBooks : Author
    {
        public Book[] AllBooks { get; set; }
    }

    public static class Program
    {
        private static async Task Main()
        {
            await DB.InitAsync("test");

            //create author and his books

            var author = new Author { Name = "Author One" };
            await author.SaveAsync();

            var books = new[] {
                new Book { Title = "Book One", Author = author },
                new Book { Title = "Book Two", Author = author }
            };

            await books.SaveAsync();

            await author.Books.AddAsync(books);

            //retrieve all books of author

            var allBooks = await author.Books
                                       .ChildrenQueryable()
                                       .ToListAsync();

            //retrieve first book of author

            var firstBook = await author.Books
                                        .ChildrenQueryable()
                                        .Where(b => b.Title == "Book One")
                                        .ToListAsync();

            //retrieve author together with all of his books

            var authorWithBooks = await DB.Fluent<Author>()
                                          .Match(a => a.ID == author.ID)
                                          .Lookup<Author, Book, AuthorWithBooks>(
                                              DB.Collection<Book>(),
                                              a => a.ID,
                                              b => b.Author.ID,
                                              awb => awb.AllBooks)
                                          .ToListAsync();
        }
    }
}