How does Basecamp 'scope' distinct users in a monolithic database?

I'm hoping to find someone who realizes the implications of this
design strategy. I am attempting to build an application that access a
common monolithic database but each user has a 'scope' so they only
see their data..
My strategy is to create an http request that looks like
project.controller.action.keyvalue and pass this throughout the
application. The "Project" term is used to describe a distinct data
scope that one user or a group would maintain.
I expect we'd make http calls like http:\
\project.controller.action.keyvalue and database calls like:
link_to: #{table.row.columname}
                                               :Project => project
                                               :Action=> action
                                               :Id => id

The patriarch of all RonR programs written by DHH (Basecamp) must
utilize a strategy similar to this as they are running the same
generic program, but accessing distinct data from one monolithic
When Basecamp passes an http: request it's got to hold inside this
package the group database, user, controller, action and value. Does
anyone know the basic architecture of how this is accomplished?
I found one clue to this question today when looking at the the
ActionPack API and in it, DHH wrote:

Routing makes pretty urls incredibly easy
  map.connect 'clients/:client_name/:project_name/:controller/:action'

  Accessing /clients/37signals/basecamp/project/dash calls
ProjectController#dash with
  { "client_name" => "37signals", "project_name" => "basecamp" } in

  From that URL, you can rewrite the redirect in a number of ways:

  redirect_to(:action => "edit") =>

  redirect_to(:client_name => "nextangle", :project_name => "rails")

I am willing to pay for someone to teach me how this is done.

Thank you,

hi dave,

my apologies if i misunderstood your question, but if you're asking
about the routing, you can set up a route like so:

    map.connect 'projects/:project_id/:controller/:action/:id'

and when you use link_to, you only have to specify :project_id on the
link that takes them into the project; subsequently, omitted
:project_id and :controller default to the current one. so once in the
widget controller for project 42, you can make links like:

  link_to , :action => "show", :id => @widget

which will create a link to: /projects/42/widget/show/23

of course on each request you need to validate that the user is a
member of the project and that the data they are asking for belongs to
the project.

hope that helps,

Take a look at this plugin for clues: