Question

I was looking at the rails 3 architecture in order to understand the process of dispatching a request . The whole process is quite simple. Application is a rack application which finally delegates its call message to the call method of ActionDispatch::Routing::RouteSet which dispatches appropriate action of necessary controller. It takes controller and action names from the hash stored in rack env by the key “action_dispatch.request.path_parameters”.

So the question is: Who sets this hash? Who parses request uri and determines action and controller names?

I was not able to find this code. I see that during route configuration ActionDispatch::Routing::Mapper object is used to construct information about defined routes and stores it in ActionDispatch::Routing::RouteSet. But how this information is used during the request to find appropriate action and controller. Is rack also somehow involved here?

Answer 1

“action_dispatch.request.path_parameters” is stored as the constant ActionDispatch::Routing::RouteSet::PARAMETERS_KEY (actionpack/lib/action_dispatch/routing/route_set.rb)

PARAMETERS_KEY is used later on in that same file and passed into the constructer for ::Rack::Mount::RouteSet

Going to ::Rack::Mount::RouteSet source here: https://github.com/josh/rack-mount/blob/master/lib/rack/mount/route_set.rb#L22 you can see that attribute name is stored.

Same file, down in the call(env) method which is of course the Rack interface that will be called into, we see this line ( https://github.com/josh/rack-mount/blob/master/lib/rack/mount/route_set.rb#L147 ) where your env["action_dispatch.request.path_parameters"] attribute is actually set, and eventually returned back into the code you were investigating.

Hope this helps!