Question

I have a Query

SpecialAttributes is a IEnumerable

 q = from c in q
     let x = c.SpecialAttributes.Where(a => a.Attribute.Id == id)
                                .Select(z =>  z.AttribValue).SingleOrDefault()
     orderby Convert.ToDateTime(x)
     select c;

I need to execute sql query at once.

But in this case each SingleOrDefault() will be executed separately of main query.

If count of fetching results will be 100 – SingleOrDefault() will be executed 100 times.

How do it in one query, like FutureSingleOrDefault() ?

Answer 1

Assuming you won’t have attributes with the same id in your special attributes collection and that your “SpecialAttribute” class has a reference to the parent object, this may work:

q.SelectMany(x => x.SpecialAttributes)
 .Where(x => x.Attribute.Id == id)
 .OrderBy(x => Convert.ToDateTime(x.AttribValue))
 .Select( x => x.Parent )
 .Distinct();

I don’t know if EF will parse the Convert.ToDateTime() to a proper t-sql statement. If not, you could do it in memory (if the collection is not huge):

q.SelectMany(x => x.SpecialAttributes)
 .Where(x => x.Attribute.Id == id)
 .Select( x => new { Parent = x.Parent, Value = x.AttribVale} )
 .Distinct()
 .OrderBy(x => Convert.ToDateTime(x.Value))
 .Select(x => x.Parent);