Entity Framework - GroupBy then OrderBy

Question

I have a group of records that includes PatientID and Appointment Date.

I would like to group them by PatientID and order the groups by the oldest appointment in each PatientID group. Like this...

 ID     ApptDate
----------------------
  3    2/5/2005   (oldest compared to all groups , so group "ID = 3" is sorted first)
  3    5/10/2006
  3    6/2/2010

  1    8/5/2007
  1    9/1/2015 

  2    6/15/2009
  2    9/19/2009        

I'm pretty sure I need to use grouping first to obtain the oldest date value for each ID and then order by that value but I am stuck understanding how the two functions will work together.

var query = from a in db.Appointments
group a by new { a.Id, a.ApptDate} into pa
select new { 
     ID = pa.Key.Id,  
     Date = pa.Min(x =>x.ApptDate) ...

... but I crash and burn at this point.

Any help appreciated.

Answer 1

You requirements are as follows (though the question does not mention one of them, but it is apparent from how data is laid out):

1. Group based on patient ID.
2. Sort groups based on oldest appointment date in each group.
3. Sort data within a group based on appointment date.

var query = records.GroupBy(r => r.PatientId) // Group patients .OrderBy(g => g.Min(r => r.AppointmentDate)) // sort groups by oldest record in each .Select(g => new { PatientId = g.Key, Records = g.OrderBy(r => r.AppointmentDate) // sort records within a group by oldest });

Answer 2

I modified a bit Tanveer Badar's answer to return a row for each entity.

Create a class for your return data

public class ReturnType
{
    public int ID { get; set; }
    public DateTime AppDate { get; set; }
}

Get a group of IDs and enumerable of ordered dates

var result = db.Appointment
            .GroupBy(a => a.Id)
            .OrderBy(a => a.Min(n => n.Date))
            .Select(a => new { ID = a.Key, AppDates = a.OrderBy(na => 
na.Date).Select(ne => ne.Date) })
            .ToList();

Then flatten the returned list. I tried with SelectMany but with no success.

var results = new List<ReturnType>();
        foreach (var a in result)
        {
            foreach (var date in a.AppDates)
            {
                var returnType = new ReturnType
                {
                    ID = a.ID,
                    AppDate = date
                };

                results.Add(returnType);
            }
        }

Answer 3

In my case it works fine...

var query = from a in db.Appointments
    group a by new { a.Id, a.ApptDate} into pa
    select new { 
         ID = pa.Key.Id,  
         Date = pa.Max(x =>x.ApptDate)
       }).Select(x=>new { 
                             x.ID,x.Date
                            }).ToList();

Answer 4

db.Appointments.GroupBy(new {ID = item.ID, ApptDate = item.ApptDate}).OrderByDescending(item => item.ApptDate)