How to do multiple joins when using groupby in lin

问题:

I have a picturerating table in my database. For every new rating ,a new picture rating table row will be created. I am grouping those ratings by lastweek and pictureid, meaning all the ratings for one single pic in the last week will be accumulated and become one rating.Then i will be able to sort them by thier count.

This is my picturerating table :

public partial class picturerating
{
     public int idpicturerating { get; set; }
     public int idpictures { get; set; }
     public int iduser { get; set; }
     public System.DateTime iddatetime { get; set; }
     public int iduserratedby { get; set; }
     public int rating { get; set; }
}

This is my code to do this :

      public IQueryable<SortedDto> GetRatignsAndTheirDetails()
    {
        var pastDate = DateTime.Now.Date.AddDays(-7);
        var combo = from p in db.picturerating
                    where p.iddatetime > pastDate
                    group p by p.idpictures into pgroup
                    let count = pgroup.Count()
                    orderby count descending
                    select new SortedDto
                    {
                        IdPictures = pgroup.FirstOrDefault().idpictures,
                        IdPicturerating = pgroup.FirstOrDefault().idpicturerating,
                        IdUser = pgroup.FirstOrDefault().iduser,
                        IdDatetime = pgroup.FirstOrDefault().iddatetime,
                        IdUserratedBy = pgroup.FirstOrDefault().iduserratedby,


                    };

        return combo;
    }

Now here comes my problem:

How to join two more tables , users and picturedetails table to show username and total rating.

As per now i am able to write this code, this is just a rough idea in my mind not a proper code so i need help to change this code to make it work:

 var pastDate = DateTime.Now.Date.AddDays(-1);
        var combo = from p in db.picturedetails
                    join u in db.users on p.iduser equals u.iduser
                    join l in db.pictureratings  where l.iddatetime > pastDate group l by l.idpictures into pgroup
                    let count = pgroup.Count()
                    orderby count descending

                    select new SortedDto
                    {
                        IdPictures = pgroup.FirstOrDefault().idpictures,
                        IdPictureRating = pgroup.FirstOrDefault().idpicturerating,
                        IdUser = pgroup.FirstOrDefault().iduser,
                        IdDatetime = pgroup.FirstOrDefault().iddatetime,
                        IdUserRatedBy = pgroup.FirstOrDefault().iduserrateddby,
                        username=u.username,
                        dob=u.dob,
                        sex=u.sex,
                        totalrating = pl.Average(l => (float?)l.rating)?? 0 , // pl.Where(a => a.ratenumber!= null).Average(c => c.ratenumber)

                    };

        return combo;

Am i doing it right, whats the way to do it, how to change the above code to make it work properly as per my needs.