lookup one to many relationship

Program.cs

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

namespace TestApplication
{
    public class Course : Entity
    {
        public string Title { get; set; }
        public One<CourseCategory> Category { get; set; }
    }

    public class CourseCategory : Entity
    {
        public string Name { get; set; }
        public Many<Course> Courses { get; set; }

        public CourseCategory()
            => this.InitOneToMany(() => Courses);
    }

    public class CourseWithCategory : Course
    {
        public IEnumerable<CourseCategory> Categories { get; set; }
    }

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

            var cat = new CourseCategory { Name = "category one" };
            await cat.SaveAsync();

            var course1 = new Course { Category = cat, Title = "first course" };
            var course2 = new Course { Category = cat, Title = "second course" };
            var courses = new[] { course1, course2 };
            await courses.SaveAsync();

            await cat.Courses.AddAsync(courses);

            //option 1: transform the reference to an entity with ToEntity
            var firstCourse = await DB
                .Find<Course>()
                .Match(c => c.ID == course1.ID)
                .ExecuteSingleAsync();

            var firstCourseCat = await firstCourse.Category.ToEntityAsync();

            //option 2: lookup the category in a single query
            var secondCourse = await DB
                .Fluent<Course>()
                .Match(c => c.ID == course2.ID)
                .Lookup<Course, CourseCategory, CourseWithCategory>(
                    DB.Collection<CourseCategory>(),
                    c => c.Category.ID,
                    cc => cc.ID,
                    cwc => cwc.Categories)
                .SingleOrDefaultAsync();

            var secondCourseCat = secondCourse.Categories.FirstOrDefault();
        }
    }
}