var lst = from c in db.Blogs join p in db.Posts on c.Id equals p.BlogId where p.Id== select c;

var lst1 = db.Blogs.Join(db.Posts.Where(p=>p.Id==), b=> b.Id, p=> p.BlogId, (b, p) => new {b});

 public class Blog

 {

    public int Id { get; set; }

    public string Title { get; set; }

 }

 public class Post

 {

    public int Id { get; set; }

    public string Title { get; set; }

    public int BlogId { get; set; }

 }

        public List<CommentsViewModel> GetCommentsByPid(int pid, int uid)

        {

            var query = from a in db.Photos

                        join b in db.Comments on a.PID equals b.PID

                        join c in db.Users on b.UID equals c.UID

                        where a.PID == pid

                        select new CommentsViewModel { Id = b.ID, Author = c.UserName, Comment = b.Content, UserAvatar = c.HeadPic, CanDelete = uid == b.UID ? true : false, CanReplay = true, TempDate = b.CreateTime, ParentId = b.ParentId };

            var ret = query.ToList();

            return ret;

       //或者

            var data = db.Photos.

                    Join(db.Comments, p => p.PID, c => c.PID,

                    (p, c) => new { p, c }).

                    Join(db.Users, n => n.c.UID, u => u.UID, (n, u) => new { n, u })

                    .Where(m => m.n.p.PID == pid)

                    .Select(m => new CommentsViewModel

                    {

                        Id = m.n.c.ID,

                        Author = m.u.UserName,

                        Comment =m.n.c.Content,

                    });

            return data.ToList();

        }