RavenDB ASP.NET Web API session management

ExampleController.cs

public class BooksApiController : FooApiController
{
    public IEnumerable<Book> Get()
    {
        return DocumentSession
            .Query<Book>()
            .OrderByAscending(p => p.Author)
            .Take(100);
    }
}

FooApiController.cs

public abstract class FooApiController : ApiController
{
    public IDocumentSession DocumentSession { get; set; }
}

Global.asax.cs

// or self-hosted startup
public class MvcApplication : HttpApplication
{
    protected void Application_Start()
    {
        ...

        var config = GlobalConfiguration.Configuration;
        config.Filters.Add(new RavenDbApiAttribute(documentStore));
    }
}

RavenDbApiAttribute.cs

public class RavenDbApiAttribute : ActionFilterAttribute
{
    readonly IDocumentStore documentStore;

    public RavenDbApiAttribute(IDocumentStore documentStore)
    {
        if (documentStore == null) throw new ArgumentNullException("documentStore");
        this.documentStore = documentStore;
    }

    public override void OnActionExecuting(HttpActionContext actionContext)
    {
        var controller = actionContext.ControllerContext.Controller as FooApiController;
        if (controller == null)
            return;

        controller.DocumentSession = documentStore.OpenSession();
    }

    public override void OnActionExecuted(HttpActionExecutedContext actionExecutedContext)
    {
        var controller = actionExecutedContext.ActionContext.ControllerContext.Controller as FooApiController;
        if (controller == null)
            return;

        controller.DocumentSession.Dispose();
    }
}

where do you get documentStore from in application_start ?