Archive done tasks and preserve tree structure

[Naresh Gurbuxani <naresh_gurbuxani@hotmail.com>]

In my task list, items are grouped under various headings.  For example:

* Reading

** TODO Book 1

** TODO Book 2

* Programming

** TODO assignment 1

** TODO assignment 2


When an item is DONE, I want to archive it such that archive file 
preserves tree structure.  So when Book 1 above is DONE, I want C-c C-x 
C-a to send this subtree to archive file under the heading Reading.  I 
followed the suggestion here:

https://stackoverflow.com/questions/10143959/keeping-the-context-when-archiving-in-emacs-org-mode

My updated tasks file looks like this.

* Reading

:PROPERTIES:
:ARCHIVE: %s_archive::Reading
:END:

** TODO Book 1

** TODO Book 2


Adding this property does not change archive action.  After marking Book 
1 as DONE, then typing C-c C-x C-a sends this subtree to archive file as 
a top level heading.  As a result, my archive file has a long list of 
top level headings.

How can I get the archiving done to preserve tree structure of tasks?  
Any hope for already archived items to be reorganized?

Thanks,

Naresh