reverse relationship access with many-to-many relationship

Program.cs

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

namespace TestApplication
{
    public class User : Entity
    {
        [OwnerSide]
        public Many<Client> Clients { get; set; }

        public User()
        {
            this.InitManyToMany(() => Clients, c => c.Users);
        }
    }

    public class Client : Entity
    {
        [InverseSide]
        public Many<User> Users { get; set; }

        public Client()
        {
            this.InitManyToMany(() => Users, u => u.Clients);
        }
    }

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

            var user = new User();
            await user.SaveAsync();

            var client = new Client();
            await client.SaveAsync();

            await user.Clients.AddAsync(client);
            await client.Users.AddAsync(user);

            //option 1 client-side lookup
            var userID = await new User().Clients
                .JoinQueryable()
                .Where(j => j.ChildID == client.ID)
                .Select(j => j.ParentID)
                .SingleOrDefaultAsync();

            var userEntity = await DB
                .Find<User>()
                .Match(userID)
                .ExecuteSingleAsync();

            //option 2 via parents queryable
            var user_Entity = await new User().Clients
                .ParentsQueryable<User>(client.ID)
                .SingleOrDefaultAsync();
        }
    }
}