Guid as Id in RavenDB

You can achieve this using a MultiMap/Reduce Index, but you will need some hackery:

1) You will need to reduce using strings, not guids. You can still get the values back as guids in your AB class, as I will demonstrate below.

2) You can't call the first property of your AB class "Id", as raven will try to translate it to "__document_id". So call it "AId" and it will work fine.

3) In the mapping phase, you have to manipulate the strings yourself to strip off the document key prefix.

Here's a sample program that puts it all together. This demonstrates that it does indeed work, but I think it also shows why Ayende prefers string identifiers so you don't have to deal with this kind of mess.

using System;
using System.Linq;
using Raven.Client.Document;
using Raven.Client.Indexes;

namespace RavenScratchTest
{
  class Program
  {
    static void Main()
    {
      var documentStore = new DocumentStore { Url = "http://localhost:8080" };
      documentStore.Initialize();
      IndexCreation.CreateIndexes(typeof(Program).Assembly, documentStore);

      using (var session = documentStore.OpenSession())
      {
        var b = new B { Id = Guid.NewGuid(), Name = "Foo" };
        var a = new A { Id = Guid.NewGuid(), BId = b.Id };

        session.Store(a);
        session.Store(b);

        session.SaveChanges();
      }

      using (var session = documentStore.OpenSession())
      {
        var a = session.Query<A>().Customize(x => x.WaitForNonStaleResults()).First();
        var b = session.Query<B>().Customize(x => x.WaitForNonStaleResults()).First();

        Console.WriteLine("A:  Id = {0}", a.Id);
        Console.WriteLine("   BId = {0}", a.BId);
        Console.WriteLine();
        Console.WriteLine("B:  Id = {0}", b.Id);
        Console.WriteLine("  Name = {0}", b.Name);
        Console.WriteLine();

        var guid = a.Id;
        var ab = session.Query<AB, MyIndex>().Customize(x => x.WaitForNonStaleResults())
          .FirstOrDefault(t => t.AId == guid);

        if (ab == null)
          Console.WriteLine("AB: NULL");
        else
        {
          Console.WriteLine("AB:  AId = {0}", ab.AId);
          Console.WriteLine("   BId = {0}", ab.BId);
          Console.WriteLine("   BName = {0}", ab.BName);
          Console.WriteLine();
        }
      }

      Console.WriteLine();
      Console.WriteLine("Done.");
      Console.ReadLine();
    }
  }

  class A
  {
    public Guid Id { get; set; }
    public Guid BId { get; set; }
  }

  class B
  {
    public Guid Id { get; set; }
    public string Name { get; set; }
  }

  class AB
  {
    public Guid AId { get; set; }
    public Guid BId { get; set; }
    public string BName { get; set; }
  }

  class MyIndex : AbstractMultiMapIndexCreationTask<MyIndex.ReduceResult>
  {
    public MyIndex()
    {
      AddMap<A>(docs => from a in docs
                select new
                {
                  AId = a.Id.ToString().Split('/')[1],
                  a.BId,
                  BName = (string)null
                });

      AddMap<B>(docs => from b in docs
                select new
                {
                  AId = (string)null,
                  BId = b.Id.ToString().Split('/')[1],
                  BName = b.Name
                });

      Reduce = results => from result in results
                group result by result.BId
                into g
                select new
                  {
                    g.FirstOrDefault(x => x.AId != null).AId,
                    BId = g.Key,
                    g.FirstOrDefault(x => x.BName != null).BName
                  };
    }

    internal class ReduceResult
    {
      public string AId { get; set; }
      public string BId { get; set; }
      public string BName { get; set; }
    }
  }
}

You can provide an ID to RavenDB explicitly upon saving:

session.Store(doc, explicitIdValueString);

The explicitIdValueString can be a Guid string. This value will be used to identify the document within the entire database and will not be prefixed by a type tag name. You can also customized the tag name, or the ID generation strategy all together by overriding conventions on IDocumentStore.Conventions such as FindTypeTagName which is a Func<Type, string>.

The main problem is that while RavenDB can deal with numeric / integer on the client, but on the server side, RavenDB uses string ids.

In general, it isn't recommended to use Guids / numeric ids.