How do you flatten a Linq query after using double grouping?

Question

Suppose I have a class representing order lines , eg

public class Line
{
    public string Code ;
    public string No ; // Invoice Number
    public DateTime Date ;
    public string Product ;
    public decimal Quantity ;
}

and a List of lines, eg

List<Line> myList = new List<Line>();
myList.Add(new Line() { Code = "ABC001", No = "1001" ,Date = new DateTime(2012,4,1) ,  Product = "X", Quantity= 1m});
myList.Add(new Line() { Code = "ABC001", No = "1001" ,Date = new DateTime(2012,4,1) ,  Product = "Y", Quantity= 1m});

myList.Add(new Line() { Code = "ABC002", No = "1002" ,Date = new DateTime(2012,4,2) ,  Product = "X", Quantity= 1m});
myList.Add(new Line() { Code = "ABC002", No = "1002" ,Date = new DateTime(2012,4,2) ,  Product = "Y", Quantity= 1m});
myList.Add(new Line() { Code = "ABC002", No = "1003" ,Date = new DateTime(2012,4,3) ,  Product = "Z", Quantity= 1m});
myList.Add(new Line() { Code = "ABC002", No = "1004" ,Date = new DateTime(2012,4,4) ,  Product = "X", Quantity= 1m});

myList.Add(new Line() { Code = "ABC003", No = "1005" ,Date = new DateTime(2012,4,4) ,  Product = "X", Quantity= 1m});
myList.Add(new Line() { Code = "ABC003", No = "1006" ,Date = new DateTime(2012,4,4) ,  Product = "X", Quantity= 1m});
myList.Add(new Line() { Code = "ABC003", No = "1006" ,Date = new DateTime(2012,4,4) ,  Product = "Y", Quantity= 1m});

I am looking to retrieve all lines where the Customer Code has more than one invoice. To do this, I am first of all grouping by the Code, No and Date and then grouping that by customer Code and for any customers which two or more records, I am selecting all but the first record.

Like so:

var query1 = 
    (from r in myList
        group r by new { r.Code ,  r.No , r.Date } into results
        group results by new { results.Key.Code } into results2 
        where results2.Count() > 1
        select new 
        {   
            results2.Key.Code , 
            Count = results2.Count(), 
            Results = results2.OrderBy(i=>i.Key.Date).Skip(1).ToList() 
          // Skip the first invoice
        } 
     ).ToList();

query1 now contains the correct records, but wrapped inside IGrouping and I am having trouble gets the results out as a List<Line>

I tried query1.SelectMany(r=>r.Results).ToList();

but this still leaves me with IGrouping and that's where I am stuck.

I could resort to the nested for loops as in

List<Line> output = new List<Line>();
foreach (var r1 in query1)
{
    foreach(var r2 in r1.Results)
        foreach(var r3 in r2)
            output.Add(r3);     
}

but is there a better/Linq way?

The actual output should be four lines as in

// Code    No    Date              Product Quantity 
// ABC002 1003 03/04/2012 00:00:00 Z 1 
// ABC002 1004 04/04/2012 00:00:00 X 1 
// ABC003 1006 04/04/2012 00:00:00 X 1 
// ABC003 1006 04/04/2012 00:00:00 Y 1 

Answer 1

You're going to kick yourself:

query1.SelectMany(q => q);

ABC002 1003 3/04/2012 12:00:00 AM Z 1 
ABC002 1004 4/04/2012 12:00:00 AM X 1 
ABC003 1006 4/04/2012 12:00:00 AM X 1 
ABC003 1006 4/04/2012 12:00:00 AM Y 1 

The return from query1 is an enumerable (I removed your lists) of IGrouping and IGrouping is itself an enumerable, so we can just flatten it directly.

See here: http://mtaulty.com/CommunityServer/blogs/mike_taultys_blog/archive/2007/09/28/9836.aspx

Edit: Remembered I also simplified your code:

var query1 = 
(from r in myList
    group r by new { r.Code ,  r.No , r.Date } into results
    group results by new { results.Key.Code } into results2 
    where results2.Count() > 1
    from result in results2.OrderBy(i=>i.Key.Date).Skip(1)
    select result
 );

Answer 2

This code:

List<Line> output = new List<Line>(); 
foreach (var r1 in query1) 
  foreach(var r2 in r1.Results)         
    foreach(var r3 in r2)
      output.Add(r3);   

Is the same thing as:

var q2 = from r1 in query1
         from r2 in r1.Results
         from r3 in r2
         select r3;
var output = q2.ToList();