My name
is
Jon Skeet

LINQ syntax for Order By largest SUM value from child collection

I have following records

User
---------------
UserID | Name
---------------
   1   | Johan
   2   | Rick
---------------

UserDetail
----------------------------------
UserDetailID | UserID | Percentage
----------------------------------
      1      |    1   |     10
      2      |    1   |     30
      3      |    2   |     50
      4      |    2   |     10
----------------------------------

Basically I want to SUM percentage per UserID and Order by the largest Total Percentage.

In TSQL :

SELECT
    User.UserID
FROM
    User
    LEFT JOIN
    (
        SELECT
            SUM(Percentage) [Total],
            UserID
        FROM
            UserDetail
        GROUP BY
            UserID
    ) qry ON
        User.UserID = qry.UserID
ORDER BY
    qry.Total DESC

My current code :

var orderedList = user.SelectMany(s => s.UserDetails).Sum(s => s.Percentage); // <-- I have no idea what I'm doing

It sounds like you don't want to use SelectMany at all - you only want one row per user. I suspect you want:

var orderedList = users.OrderByDescending(user => user.UserDetails.Sum(d => d.Percentage));

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