Complex query

Hi,
Part of my Invoice table looks like this:

create_table “invoices”, :force => true do |t|
t.datetime “sent_at”,
t.datetime “received_payment_at”,
t.integer “total_fee”

end

I want to put together view with monthly sums for ‘invoiced amounts’ and
‘received amounts’ like this:

month | total invoiced | total received
Feb 08 | 20,000 | 15,000
Mar 08 | 35,000 | 30,000

What’ the best way to write a query & a view to achieve this?

Thanks!
RQ