activeresource: self.prefix = .../:id/... (need to set :id?)

Hi All

As I mentioned in my title I'm using activeresource and the model I'm
using has the following prefix

self.prefix = '/institute/:institute_id/services/:service_id/'

All this is great, however I'm now in a situation where I need to save
the model, but it translate the prefix as follows

/providers/983783784/services//appointments/234234234

So, the :service_id is unknown and is not part of the prefix.

Here is the save command:

@myModel.save(:params => {:a => {:service => %w(mylink)}, :user_id =>
current_user.id})

I'm not an activeresource expert so I'm not really sure whats going on
here.
Is :a something activeresource understands (or maybe a hack of the
application) ?

Furthermore, suppose I have the service_id in a variable (@service_id),
how do I add this to this method ?
I've tried things like:

@myModel.save(:params => {:a => {:service => %w(mylink)}, :user_id =>
current_user.id, :service_id => 123})

but it didn't fix it (I guess the stuff with the :a doesn't work)

Thnx a lot
Luca

ps: links to useful docs would be helpful too!!