Question

I’m using Lucene.net to build a MyListOfIds As List(Of Integer) which I then pass on to my Linq service. I then search the database as follows

Return _EventRepository.Read().Where(Function(e) MyListOfIds.Contains(e.ID)).ToList

Now I know that Lucene is already ordering MyListOfIds based on the weight it gave each term. What sucks is that Linq is losing that order in it’s SQL search.

My Question: How can I retain that sort order when building my Lambda expression?

I tried using LINQPad to see how the query is being built, but because I had to declare a variable LINQPad didn’t show me the resultant SQL 🙁

Here’s what I tried in LINQPad

Dim i As New List(Of Integer)
i.Add(1)
i.Add(100)
i.Add(15)
i.Add(3)
i.Add(123)

Dim r = (From e In Events
         Where i.Contains(e.ID)
         Select e)

note: my example is in VB.NET, but I don’t mind if responses are in C#

Answer 1

As hangy mentioned, I think the Dictionary method is the way to go. I’d go about it this way:

Public Function GetLuceneSearchResults(ByVal ids As List(Of Integer)) As List(Of Domain.Event) Implements IEventService.GetLuceneSearchResults
    Dim Results = (From e In _EventRepository.Read()
                   Where ids.Contains(e.ID)
                   Select e).ToDictionary(Function(e) e.ID, Function(e) e)

    Return (From i In ids
            Where Results.ContainsKey(i)
            Select Results(i)).ToList()
End Function

The first query returns a Dictionary with the event id as key, and the event itself as a value. You get the performance benefit of a hash lookup this way.