I am a Cloud subscriber setting up a basic blog. I want to filter the list of blog articles by author, but I am having a hard time with querying a collection on the value of a property that has properties of its own. "Author" is a document type. "Article" is a document type that has a custom ContentPicker for selecting an "Author". I have the following code in my view template:

var articles = Umbraco.Content(Guid.Parse("654c40f6-187e-4fa7-a68e-00ce50e7aa9c"))
        .Children<Article>("article")
        .Where(x => x.IsVisible())
        .OrderByDescending(x => x.Value<DateTime>("publishDate"));


var requestQueryString = String.Empty;

// If author parameter is selected, narrow articles to author in Request query author parameter
if(!string.IsNullOrEmpty(HttpContextAccessor.HttpContext.Request.Query["author"]))
{
    requestQueryString = HttpContextAccessor.HttpContext.Request.Query["author"];

    // Everything works until I try to filter by author
    articles = articles.Where(a => a.Author.Key == requestQueryString);
}

I'm not sure how I would achieve this in Umbraco. Any insights are greatly appreciated. Thank you. :)