RavenDB Querying on ID

I have a Model called Post, which has a field called Slug. I've marked this field with the attribute [ID] and changed the FindIdentityProperty Convention in RavenDB such that it considers the property marked with [ID] as the Identity of the document.

Now I want to fetch all posts that have slugs that start with the character G, do I need to create an index like this :

from Post in docs.Posts
select new { Slug = Post.Slug }

or like this

from Post in docs.Posts
select new { Id = Post.Id }

or do I use the index that RavenDB internally should have since I'm querying on ID. I understand that RavenDB has the ID property indexed somewhere so that a Session.Load<Post>("SomeID") is possible.

The 1st option can be defined in code in C# as an AbstractIndexCreationTask because C# understands that Slug is a field of Post, but does not work as it is "string"ified into RavenDB like this : (when Index Creation happens)

from Post in docs.Posts
select new { Slug = Post.Slug }

becomes

docs.Posts
    .Select(Post => new {Slug = Post.Slug })

You can see this when you look in the SL-UI, after indexes are created.

And this does NOT work because ( as shown in the SL-UI of the DB ) the JSON representation of the Post, there is no name-value pair like "Slug":"Some-Slug". Since it's the ID it is shown above the document, and in the @metadata it can be seen as the value of _document_id

The latter one does work, but only when defined as a string, like this in C#

public class Po开发者_StackOverflowst_ById : AbstractIndexCreationTask
    {
        public override IndexDefinition CreateIndexDefinition()
        {
            return new IndexDefinition
                       {
                           Map = "from Post in docs.Posts select new { Id = Post.Id}"
                       };
        }
    }

I suspect when Raven gets queries that contain the string "Id", it automatically takes it to mean that it's a query on [@metadata][_document_id].

No LINQ goodness here - looks very brittle, won't survive a refactor of the domain name. Id is not a field of Post, but this index did allow me to do what I wanted, namely, "fetch all posts that have slugs that start with the character G". And I hate defining indexes for Ids when DB should already have such an index.

Hm, we actually use the same conventions in AbstractIndexCreationTask as well, so that should work. At any rate, the id of the document can always be accessed using __document_id