LINQ - Previous Record - linq

Question

All:
Lets say I have the following table:
RevisionID, Project_ID, Count, Changed_Date
1 2 4 01/01/2016: 01:02:01
2 2 7 01/01/2016: 01:03:01
3 2 8 01/01/2016: 01:04:01
4 2 3 01/01/2016: 01:05:01
5 2 15 01/01/2016: 01:06:01
I am ordering the records based on Updated_Date. A user comes into my site and edits record (RevisionID = 3). For various reasons, using LINQ (with entity framework), I need to get the previous record in the table, which would be RevisionID = 2 so I can perform calculations on "Count". If user went to edit record (RevisionID = 4), I would need to select RevisionID = 3.
I currently have the following:
var x = _db.RevisionHistory
.Where(t => t.Project_ID == input.Project_ID)
.OrderBy(t => t.Changed_Date);
This works in finding the records based on the Project_ID, but how then do I select the record before?
I am trying to do the following, but in one LINQ statement, if possible.
var itemList = from t in _db.RevisionHistory
where t.Project_ID == input.Project_ID
orderby t.Changed_Date
select t;
int h = 0;
foreach (var entry in itemList)
{
if (entry.Revision_ID == input.Revision_ID)
{
break;
}
h = entry.Revision_ID;
}
var previousEntry = _db.RevisionHistory.Find(h);

Answer 1

Here is the correct single query equivalent of your code:
var previousEntry = (
from r1 in db.RevisionHistory
where r1.Project_ID == input.Project_ID && r1.Revision_ID == input.Revision_ID
from r2 in db.RevisionHistory
where r2.Project_ID == r1.Project_ID && r2.Changed_Date < r1.Changed_Date
orderby r2.Changed_Date descending
select r2
).FirstOrDefault();
which generates the following SQL query:
SELECT TOP (1)
[Project1].[Revision_ID] AS [Revision_ID],
[Project1].[Project_ID] AS [Project_ID],
[Project1].[Count] AS [Count],
[Project1].[Changed_Date] AS [Changed_Date]
FROM ( SELECT
[Extent2].[Revision_ID] AS [Revision_ID],
[Extent2].[Project_ID] AS [Project_ID],
[Extent2].[Count] AS [Count],
[Extent2].[Changed_Date] AS [Changed_Date]
FROM [dbo].[RevisionHistories] AS [Extent1]
INNER JOIN [dbo].[RevisionHistories] AS [Extent2] ON [Extent2].[Project_ID] = [Extent1].[Project_ID]
WHERE ([Extent1].[Project_ID] = #p__linq__0) AND ([Extent1].[Revision_ID] = #p__linq__1) AND ([Extent2].[Changed_Date] < [Extent1].[Changed_Date])
) AS [Project1]
ORDER BY [Project1].[Changed_Date] DESC

Answer 2

hope I understood what you want.
Try:
var x = _db.RevisionHistory
.FirstOrDefault(t => t.Project_ID == input.Project_ID && t.Revision_ID == input.Revision_ID -1)
Or, based on what you wrote, but edited:
_db.RevisionHistory
.Where(t => t.Project_ID == input.Project_ID)
.OrderBy(t => t.Changed_Date)
.TakeWhile(t => t.Revision_ID != input.Revision_ID)
.Last()