[storm] Querying using a 'where' condition that referes to a Reference. How to?

[Mac Ryan]

Hello everybody,

	sorry to ask something that maybe has been asked already, but I am
struggling a bit to find my way alone with the limited documentation
available.

SAMPLE SITUATION (pseudocode):

        table "authors"
          name = ...
          country = ...
        
        table "books"
          title = ...
          author = Reference("authors")

DESIDERATA:

I would like to select all the books written by authors from a given
country. I believed it would be possible to do:

store.find(Books, Books.author.country == 'mycountry')

but this doesn't work. Which is the "storm-correct" way of doing this?
[I thought about the join syntax and I found this:
http://twistedmatrix.com/users/radix/storm-api/storm.expr.Join.html 
but unluckily that is undocumented]

Any advice?
Thank you in advance for your time,
Mac.