Context: I am making an e-learning app in ASP.NET with MongoDB

I have a Course collection

public class CourseDTO : DTOBase
{
    [BsonId] public ObjectId Id { get; init; }
    public required string Name { get; init; }
    public required string Description { get; init; }
    public required string ImageUrl { get; init; }
    public required ObjectId Teacher { get; init; }
    public ObjectId[] StudentIds { get; init; } = [];
}

and a User collection

public class UserDTO : DTOBase
{
    [BsonId] public ObjectId Id { get; init; }
    public required string Username { get; init; }
    public required string PasswordHash { get; init; }
    public required string PasswordSalt { get; init; }
}

I want to implement a method to fetch a Course from its ObjectId and map the StudentIds to the related Users

If possible I would prefer to do it in a single request, and keep the query type safe

So far I managed to fetch the Course with the “easy” fields, I am just missing the Students

public async Task<CourseModel> Get(ObjectId id) {
    var query = from c in repo.Courses.AsQueryable()
        where c.Id == id
        join t in repo.Users.AsQueryable() on c.Teacher equals t.Id
        select new CourseModel {
            Id = c.Id.ToString(),
            Name = c.Name,
            Description = c.Description,
            ImageUrl = c.ImageUrl,
            Teacher = new UserModel {
                Id = t.Id.ToString(),
                Username = t.Username
            }
            // todo: add students
        };

    var course = await query.FirstOrDefaultAsync();
    if (course == null) throw new NotFoundException("Course not found");

    return course;
}

public class UserModel
{
    public required string Id { get; init; }
    public required string Username { get; init; }
}

public class CourseModel
{
    public required string Id { get; init; }
    public required string Name { get; init; }
    public required string Description { get; init; }
    public required string ImageUrl { get; init; }
    public required UserModel Teacher { get; init; }
    public UserModel[] Students { get; init; } = [];
}

I could get the Students in another query easily but it would be more efficient to do everything in one query