LINQ to SQL with multiple joins, group by and count - linq-to-sql

Question

I have 3 tables, Cust, Order and Item with the following relevant fields:
Cust - CustID, CustName
Order - OrderID, CustID
Item - ItemID, OrderID
I want to find the total number of orders and items for each customer.
Here is an SQL statement that generates what I want, a list of customers with the total number of orders for each customer and the total number of items ordered.
SELECT
Cust.CustID, Cust.CustName,
count(DISTINCT Order.OrderID) AS numOrders,
count(DISTINCT Item.ItemID ) AS numItems
FROM Cust
LEFT JOIN Order ON Order.CustID = Cust.CustID
LEFT JOIN Item ON Item.OrderID = Order.OrderID
GROUP BY Cust.CustID, Cust.CustName
ORDER BY numItems
My first attempt at converting this to LINQ was to just count items and came up with this:
var qry = from Cust in tblCust
join Order in tblOrder on Cust.CustID equals Order.CustID
join Item in tblItem on Order.OrderID equals Item.OrderID
group Cust by new {CustID = Cust.CustID, CustName = Cust.CustName} into grp
orderby grp.Count()
select new
{
ID = grp.Key.CustID,
Name = grp.Key.CustName,
Cnt = grp.Count()
};
With this code I get the exception:
Value cannot be null. Parameter name: inner
Am I on the right track? What would I have to do to get both counts?

Answer 1

For Left Joins - I suggest using a from with a where and a DefaultIfEmpty
You need to group using an anonymous type in order to group multiple parameters
Value cannot be null. Parameter name: inner
Are any of joining properties nullable?
var qry =
from Cust in tblCust
from Order in tblOrder.Where(x => Cust.CustID == x.CustID)
.DefaultIfEmpty()
from Item in tblItem.Where(x => Order.OrderID == x.OrderID)
.DefaultIfEmpty()
group new { Cust, Order.OrderId, Item.ItemId } by new { Cust.CustID, Cust.CustName } into grp
let numItems = grp.Select(x => x.ItemId).Distinct().Count()
orderby numItems
select new
{
ID = grp.Key.CustID,
Name = grp.Key.CustName,
numOrders = grp.Select(x => x.OrderId).Distinct().Count(),
numItems,
};