Need help converting a Linq query (Silverlight RIA Entity - Oracle DB) - oracle

Question

I have no problem with SQL, but I'm finding Linq a little confusing.
C#, .NET4, Silverlight, RIA services, Oracle DB (v?), VS2010 running Devart dotConnect 6.10.121.
I have a RIA Entity
public sealed partial class ProcessLogHdr : Entity
{
DateTime JobDate;
string InterfaceName;
int SuccessfulCount;
int FailCount;
int TotalCount;
}
There are more fields such as user, etc, that won't be applicable to this post.
There are many jobs that make up a process. Each job has an entry in this table, but the view I want is a date group by summary.
I will be calling context.Load on a query, where I pass in the start and end date, which in Oracle looks like this:
select
trunc(JobDate),
InterfaceName,
sum(SuccessfulCount) as Total_Pass,
sum(FailCount) as Total_Fail,
sum(TotalCount) as Total,
max(JobDate) as Last_Msg_Processed_At_DT
from
ProcessLogHdrsEntity
where
JobDate >= START_DATE_IN_VAR and
JobDate <= END_DATE_IN_VAR
group by
trunc(JobDate),
InterfaceName
order by
trunc(JobDate) desc,
InterfaceName asc;
conttext.Load will call the linq query from a method that returns IQueryable.
The linq statement must run for Oracle under Devart dotConnect for Oracle.
I'm guessing I need some custom class to hold the results, like ProcessLogHdrDateSummary.
If you guys could help me fill in the missing ????? linq, I would be so grateful:
public IQueryable<ProcessLogHdrDateSummary> GetProcessLogHdrsDateSummary(DateTime START_DATE_IN_VAR, DateTime END_DATE_IN_VAR)
{
return ?????
}
Many Thanks!

Answer 1

There's no simple answer to this. One of the characteristics of LINQ to database providers is that some queries execute immediately, while others don't. Aggregation functions (MAX, MIN, etc) return immediately. So do some LINQ functions that specify particular output, such as .First(). Anything returning a collection will likely not execute immediately, and will return an IQueryable<> of some type.
What type? That depends on what the select clause of the LINQ statement specifies (which is not the same as the generated SQL select clause). "from c in db.customers select c" returns customer objects, but you can also use the select clause to populate other classes, or anonymous classes.
If a LINQ query returns an IQueryable<>, remember that the query hasn't executed yet! It won't execute until you start processing the data. You must process the data while still in scope of the data context, because once that's gone, you've lost your database connection.
You can always force an IQueryable<> to execute by ending it with .ToList(), .ToArray(), .ToDictionary(), or a few others. The List<> will use the same generic type as the IQueryable<> and the select clause (or .Select() method) of the LINQ statement.

Answer 2

The LINQ query will be rather complicated. I recommend that you follow these steps:
1. Create a stored procedure with an out cursor:
CREATE PROCEDURE myQuery(
DATE START_DATE_IN_VAR,
DATE END_DATE_IN_VAR,
cur out sys_refcursor) AS
BEGIN
OPEN cur FOR SELECT
trunc(JobDate),
InterfaceName,
sum(SuccessfulCount) as Total_Pass,
sum(FailCount) as Total_Fail,
sum(TotalCount) as Total,
max(JobDate) as Last_Msg_Processed_At_DT
from
ProcessLogHdrsEntity
where
JobDate >= START_DATE_IN_VAR and JobDate <= END_DATE_IN_VAR
group by
trunc(JobDate), InterfaceName
order by
trunc(JobDate) desc, InterfaceName asc;
END;
2. Add this procedure to the model. If you are using Devart Entity model, the return type will probably be generated. In other case you will need to create an entity or a complex type that will represent the return type of the generated method.
3. Treat the method call as a usual DomainService method.