Elegancy challenge: Cacheable Batches

Let us say that we have the following server code:

public JsonDocument[] GetDocument(string[] ids)
{
  var  results = new List<JsonDocument>();
  foreach(var id in ids)
  {
    results.Add(GetDocument(id));
  }
  return result.ToArray();  
}

This method is a classic example of batching requests to the server. For the purpose of our discussion, we have a client proxy that looks like this:

public class ClientProxy : IDisposable
{
  MemoryCache cache = new MemoryCache();
  
  public JsonDocument[] GetDocument(params string[] ids)
  {
    
    // make the request to the server, for example
    var request = WebRequest.Create("http://server/get?id=" + string.Join("&id=", ids));
    using(var stream = request.GetResponse().GetResposeStream())
    {
      return  GetResults(stream);
    }
  }
  
  public void Dispose()
  {
    cache.Dispose();
  }
}

Now, as you can probably guess from the title and from the code above, the question relates to caching. We need to make the following pass:

using(var proxy = new ClientProxy())
{
  proxy.GetPopularity("ayende", "oren"); // nothing in the cahce, make request to server
  proxy.GetPopulairty("rhino", "hippo"); // nothing in the cahce, make request to server
  proxy.GetPopulairty("rhino", "aligator"); // only request aligator, 'rhino' is in the cache
  proxy.GetPopulairty("rhino", "hippo");  // don't make any request, serve from cache
  proxy.GetPopulairty("rhino", "oren");   // don't make any request, serve from cache
}

The tricky part, of course, is to make this elegant. You can modify both server and client code.

I simplified the problem drastically, but one of the major benefits in the real issue was reducing the size of data we have to fetch over the network even for partial cached queries.