Агрегат LINQ по нескольким таблицам

Question

Я хочу повторить этот запрос в LINQ to SQL, но слишком незнаком с тем, как это сделать.

SELECT A.Recruiter, SUM(O.SaleAmount * I.Commission)  --This sum from fields in two different tables is what I don't know how to replicate
FROM Orders AS O
INNER JOIN Affiliate A ON O.AffiliateID = A.AffiliateID
INNER JOIN Items AS I ON O.ItemID = I.ItemID
GROUP BY A.Recruiter

У меня так далеко:

from order in ctx.Orders
join item in ctx.Items on order.ItemI == item.ItemID
join affiliate in ctx.Affiliates on order.AffiliateID == affiliate.AffiliateID
group order  //can I only group one table here?
  by affiliate.Recruiter into mygroup
select new { Recruiter = mygroup.Key, Commission = mygroup.Sum(record => record.SaleAmount * ?????) };

Answer 1

попробуйте linqpad, просто Google, потрясающий инструмент!

Answer 2

group new {order, item} by affiliate.Recruiter into mygroup 
select new {
  Recruiter = mygroup.Key,
  Commission = mygroup
    .Sum(x => x.order.SaleAmount * x.item.Commission)
}; 

И альтернативный способ написания запроса:

from aff in ctx.Affiliates
where aff.orders.Any(order => order.Items.Any())
select new {
  Recruiter = aff.Recruiter,
  Commission = (
    from order in aff.orders
    from item in order.Items
    select item.Commission * order.SaleAmount
    ).Sum()
};