gistfile1.cs

public class Forum
{
  public ObjectId Id{get;set;}
  public string Name{get;set;}
  //no reference to all the posts
}

public class Thread
{
  public ObjectId Id{get;set;}
  public string Title{get;set;}
  public string Body{get;set;}
  public IList<Reply> Replies{get;set;}
  public ObjectId ForumId{get;set;}
}

public class Reply
{
   public ObjectId Id{get;set;}
   public string Body{get;set;}
   public ObjectId User_Id{get;set;}
}