How to order by last modified in RavenDB?

I'm trying to allow my entities to be ordered by the Last-Modified property in their metadata, using OData query options.

I tried using a transformer as described in Converting to JSON and accessing metadata, but when I apply ODataQueryOptions to the resulting IQueryable, I get an empty array.

The model and view-model:

public class Foo
{
    public int Id { get; set; }
}

public class FooViewModel
{
    public string Id { get; set; }

    public DateTime LastModified { get; set; }
}

The transformer:

public class Foos_WithLastModified : AbstractTransformerCreationTask<Foo>
{
    public Foos_WithLastModified()
    {
        TransformResults = foos => from foo in foos
            let metadata = MetadataFor(foo)
            select new
            {
                Id = foo.Id.ToString(CultureInfo.InvariantCulture),
                LastModified = metadata.Value<DateTime>("Last-Modified")
            };
    }
}

The relevant method in FooController (_session is an IAsyncDocumentSession):

public async Task<ICollection<FooViewModel>> Get(ODataQueryOptions<FooViewModel> options)
{
    var settings = new ODataValidationSettings();
    settings.AllowedOrderByProperties.Add("LastModified");

    options.Validate(settings);

    var foos = _session.Query<Foo>()
        .TransformWith<Foos_WithLastModified, FooViewModel>();
    var odataFoos = (IQueryable<FooViewModel>)options.ApplyTo(foos);
    return await odataFoos.ToListAsync();
}

When I hit /api/Foo, the results are as expected:

[
  {
    "Id": "foos/456",
    "LastModified": "2015-11-23T08:43:10.913662Z"
  },
  {
    "Id": "foos/123",
    "LastModified": "2015-11-23T08:50:34.0907996Z"
  }
]

But when I add OData query options (/api/Foo?$orderby=LastModified), I get an empty array: [].

I also tried changing _session to an IDocumentSession and modifying Get as follows,

[EnableQuery(AllowedOrderByProperties = "LastModified")]
public IQueryable<FooViewModel> Get()
{
    return _session.Query<Foo>()
        .TransformWith<Foos_WithLastModified, FooViewModel>();
}

but I get the same results.

Are transformers the wrong approach? How can I sort by Last-Modified using OData query options?

Solution

I do not know how to handle the OData stuff, never tried that, but in order to query for entities, ordered by the metadata value "Last-Modified" using only RavenDB techniques you can do the following:

Create an index for your entity (in my example a Customer). In this index we add the field LastModified that's using the document's metadata value for Last-Modified.

public class Customer_ByLastModified : AbstractIndexCreationTask<Customer>
{
    public class QueryModel
    {
        public DateTime LastModified { get; set; }
    }

    public Customer_ByLastModified()
    {
        Map = customers => from customer in customers
            select new
            {
                LastModified = this.MetadataFor(customer).Value<DateTime>("Last-Modified")
            };
    }
}

The QueryModel isn't mandatory, but it makes querying via the client API easier, imo. You can then add a Transformer to be able to use the metadata value in your return model:

public class Customers_WithLastModified : AbstractTransformerCreationTask<Customer>
{
    public Customers_WithLastModified()
    {
        TransformResults = results => from customer in results
            select new CustomerViewModel
            {
                Id = customer.Id,
                Name = customer.Name,
                LastModified = MetadataFor(customer).Value<DateTime>("Last-Modified")
            };
    }
}

And then query it like this:

using (var session = documentStore.OpenSession())
{
    var customers = session.Query<Customer_ByLastModified.QueryModel, Customer_ByLastModified>()
        .OrderByDescending(x => x.LastModified)
        .TransformWith<Customers_WithLastModified, CustomerViewModel>()
        .ToList();
}

Hope this helps!