C# and LINQ: ordering a query by a nested query's value

I'm writing a simple forum in ASP.NET that sits on top of an Entity Framework database in C#.

Each Topic object has a navigation property Posts pointing to a collection of Post objects. Each Post object has a property When that indicates when the post was made.

Post.Parent is type Topic. Post.Identifier and Topic.Identifier is type Int32. Only Post has a When property; Topic has no such property. Topic.Parent is a third type Forum that is referenced by its Identifier.

My problem is th开发者_运维百科is: I can't seem to find a way to sort all Topic objects by the last post made in each topic. I've tried this:

var topics = from t in context.Topics
             from p in a.Posts
             where t.Parent.Identifier == forum.Identifier
             orderby p.When descending
             select t;

But I got duplicate Topic objects and sorting was not by latest post date descending.

I then tried this:

var topics = (from t in context.Topics
              let lastPost =
                  (from p in context.Posts
                   where p.Parent.Identifier == a.Identifier
                   orderby p.When descending
                   select p).FirstOrDefault().When
              where t.Parent.Identifier == forum.Identifier
              orderby lastPost descending
              select t).Distinct();

It eliminated the duplicate problem, but still no sorting. I even tried a suggestion from another question:

var topics = (from t in context.Topics
              let posts = context.Posts.Where(p => p.Parent.Identifier == t.Identifier)
              let lastPost = posts.OrderByDescending(p => p.When).FirstOrDefault()
              where t.Parent.Identifier == forum.Identifier
              orderby lastPost.When descending
              select t);

Not sure what to try next; it seems that these more advanced LINQ expressions escape me. Any ideas?

This should work:

var topics = from t in context.Topics
             where t.Parent.Identifier == forum.Identifier
             let lastPost = t.Posts.OrderByDescending(p => p.When).First()
             orderby lastPost.When descending
             select t;

(assuming a Topic has always one or more Post)