Linq-to-Entity 左 JOIN

这是我的查询：

from forum in Forums
    join post in Posts on forum equals post.Forum into postGroup    

    from p in postGroup     
    where p.ParentPostID==0

    select new 
    {
        forum.Title,
        forum.ForumID,  
        LastPostTitle = p.Title,
        LastPostAddedDate = p.AddedDate         
    }).OrderBy(o=>o.ForumID) 

目前加入不是左加入，这意味着如果某个论坛没有属于它的帖子，则不会返回该帖子。
没有帖子的论坛必须返回帖子属性的 null（或默认）值。

UPDATE

结果集应该是这样的：

ForumId | ForumTitle | LastPostTitle | LastPostAddedDate  
--------+------------+---------------+------------------
4       |   Sport    |    blabla     |       12/4/2010  
4       |   Sport    |    blabla     |       15/4/2010  
6       |   Games    |    blabla     |       1/5/2010  
7       |   Flame    |               |

 var allforums = from f in context.Fora.Include("Posts")
                           select f;

该查询产生与以下相同的结果

            var allForums = from f in context.Fora  
                            select new ForumPosts  
                            {  
                                Forum = f,  
                                Posts = context.Posts.Where(x=> x.ForumId == f.ForumId)