Convert SQL subselect to LINQ - linq

Question

I have a SQL subselect query that will retrieve the newest history record for a parent record.
I am having trouble converting the following to LINQ:
SELECT *
FROM ProductRequests INNER JOIN ProductRequestHistory
ON ProductRequests.ID = ProductRequestHistory.ProductRequestsID
AND ProductRequestHistory.ID = (SELECT TOP 1 ProductRequestHistory.ID
FROM ProductRequestHistory
WHERE ProductRequestHistory.ProductRequestsID = ProductRequestHistory.ID
ORDER BY ProductRequestHistory.DateCreated DESC)
This is what I have tried, but it doesn't compile:
from productrequests in db.ProductRequests
join productrequesthistories in db.ProductRequestHistories
on new { productrequests.ID, Column1 = (Int32?)Convert.ToInt32(
((from productrequesthistories0 in db.ProductRequestHistories
where
productrequesthistories0.ProductRequestsID == productrequesthistories0.ID
orderby
productrequesthistories0.ID descending
select new {
productrequesthistories0.ID
}).Take(1).First().ID)) }
equals new { ID = (System.Int32?)productrequesthistories.ProductRequestsID, Column1 = productrequesthistories.ID }
select new {
productrequests.ID,
productrequests.ProductRequestNumber,
productrequests.ProjectID,
Column1 = productrequesthistories.ID,
productrequesthistories.Title,
productrequesthistories.Requester,
productrequesthistories.TFSNumber,
productrequesthistories.UseCase,
productrequesthistories.RequestType,
productrequesthistories.FunctionalModule,
productrequesthistories.Description,
productrequesthistories.Reason,
productrequesthistories.Priority,
productrequesthistories.Status,
productrequesthistories.Release,
productrequesthistories.Estimate,
productrequesthistories.Wags,
productrequesthistories.Confidence,
productrequesthistories.Notes,
productrequesthistories.RequirementStatus,
productrequesthistories.RequirementDoco,
productrequesthistories.AlphaRequired,
productrequesthistories.Sprint,
productrequesthistories.Created,
productrequesthistories.DateChanged,
productrequesthistories.Checksum,
productrequesthistories.ProductRequestsID
}

Answer 1

This in VB.NET
Dim result = From request in ProductRequests
Join request2 in ProductRequestHistory
On request.ID = request2.ProductRequestsID
And request2.ID = (From request3 in ProductRequestHistory
Where request3.ProductRequestsID = request3.ID
Order by request3.DateCreated desc
Select request3.ID).First()
Select New With {
.request = request
.history = request2
}
In C#
var result = (from request in db.ProductRequests
Join request2 in ProductRequestHistory
On request.ID == request2.ProductRequestsID
&& request2.ID == (from request3 in ProductRequestHistory
where request3.ProductRequestsID == request3.ID
orderby request3.DateCreated desc
select request3.ID).First()
select new {
request = request
history = request2
}
)
That should do the trick.
Also, in the original SQL statement, I raccomend to change the
AND ProductRequestHistory.ID = (SELECT TOP 1 ProductRequestHistory.ID ......
part to a WHERE clause, since it's not necessary to the JOIN.
So, in the LINQ you can use
Where request2.ID = (From request3 in ProductRequestHistory ......
Moreover, you can optimize the query pre-calculating the TOP 1 ID value:
top1id = (From request3 in ProductRequestHistory
Where request3.ProductRequestsID = request3.ID
Order by request3.DateCreated desc
Select request3.ID).First()
And then use Where request2.ID = top1id.

Answer 2

If I am not mistaken:
var rusult = ProductRequests.Join(ProductRequestHistory,
p => p.ID,
d => d.ProductRequestsID,
(p, d) => p).where(w => w.Id == ProductRequestHistory.OrderByDescending(o=>o.DateCreated).FirstOrDefault(s=>s.ProductRequestsID == s.ID));