GroupBy, Sum and Count using linq

Question

I am trying to get a throughput view where I am calculating some stats from one table in MVC 5 using linq queries. I need to group the rows by date and count and sum of some columns

//model

public class Assignment
{
    public int UserId { get; set; }
    public Datetime Modified {get; set; } 
    public double TimeSpent { get; set; }}
    public string date
    {
        get
        {
            return Modified.Date.ToString("d");
        }
    }
}

//controller

public ActionResult Throughputs()
{
    var records = db.Assignments.AsEnumerable(); 
    var groups = from r in records
                 group r by r.date
                 into row
                 select new ThroughputsViewModel
                 {
                     selectedDate = row.Key,
                     Total = row.Where(x => x.TimeSpent > 0).Select(x => x.UserId).Count(),
                     Total_Time = row.Sum(x => x.TimeSpent),
                     Rate = row.Where(x => x.TimeSpent > 0).Select(x => x.UserId).Count() / row.Sum(x => x.TimeSpent),
                     ResAssignments = row.AsEnumerable()
                 };
    return View(records);
}

//ViewModel

public class ThroughputsViewModel
{
    public int Total { get; set; }
    public double Total_Time { get; set; }
    public double Rate { get; set; }
    public virtual User user { get; set;  }
    public int userId { get; set;  }
    public string selectedDate { get; set; }
    public IEnumerable<Assignment> ResAssignments { get; set; }
}

When I run this either I get a nullException or the rows are not grouped by the key just the same as in the original table and the sum and count values are duplicated for each row