Group by most recent date

Question

I'm relatively new to LINQ but well versed in SQL itself. I have a query where I'm trying to join 2 tables - one a list of cars and the other a record of test drives on each. I figured how to join with the most recent test but am not getting the comments field specific to that.

So lets say that the tests data are:

CarID   |TestDriveDate          |Comments
========+=======================+===========
1       |1/7/2019               |ok
2       |12/7/2019              |average
2       |10/7/2019              |poor
2       |15/7/2019              |good

With the following code:

var query = from test in TestDrives
                  group test by test.CarID into g
                  join car in Cars
                  on g.Key equals car.CarID

                  select new TestViewModel
                  {
                      CarID = g.Key,
                      CarName = car.CarName
                      Model = car.Model
                      LastTestDrive = g.Max(a => a.TestDriveDate),
                      //and the comment field where its the max inspection date
                      Comments = g.Select(a => a.Comments).FirstOrDefault()
                  };

So the following record should be returned:

CarID   |TestDriveDate          |Comments
========+=======================+===========
2       |15/7/2019              |good

Answer 1

You don't need to change your Group , you need to add a Sort .

Comments = g.Select(a => a.Comments).Sort((test1, test2)=>test1.TestDriveDate.CompareTo(test2.TestDriveDate)).FirstOrDefault();

Or add the orderby to the select .

var query = from test in TestDrives
                  orderby test.TestDriveDate
                  group test by test.CarID into g
                  join car in Cars
                  on g.Key equals car.CarID