Question

I have two classes: Property and PropertyValue. A property has several values where each value is a new revision.

When retrieving a set of properties I want to include the latest revision of the value for each property.

in T-SQL this can very efficiently be done like this:

SELECT 
p.Id,
pv1.StringValue,
pv1.Revision
FROM dbo.PropertyValues pv1
LEFT JOIN dbo.PropertyValues pv2 ON pv1.Property_Id = pv2.Property_Id AND pv1.Revision < pv2.Revision
JOIN dbo.Properties p ON p.Id = pv1.Property_Id
WHERE pv2.Id IS NULL
ORDER BY p.Id

The “magic” in this query is to join on the lesser than condition and look for rows without a result forced by the LEFT JOIN.

How can I accomplish something similar using LINQ to EF?

The best thing I could come up with was:

from pv in context.PropertyValues
group pv by pv.Property into g
select g.OrderByDescending(p => p.Revision).FirstOrDefault()

It does produce the correct result but is about 10 times slower than the other.

Answer 1

Maybe this can help. Where db is the database context:

(
            from pv1 in db.PropertyValues
            from pv2 in db.PropertyValues.Where(a=>a.Property_Id==pv1.Property_Id && pv1.Revision<pv2.Revision).DefaultIfEmpty()
            join p in db.Properties
                on pv1.Property_Id equals p.Id
            where pv2.Id==null
            orderby p.Id
            select new
            {
                p.Id,
                pv1.StringValue,
                pv1.Revision
            }
    );