Pimping a Controller

I've recently started working on a REST based rails project. I've been thinking about the Rails model associations, and in particular has_many :through, which is used to help expose the join between two models. Given the following three models:

class User < ActiveRecord::Base
  has_many :permissions
  has_many :roles, :through => :permissions
end
class Permission < ActiveRecord::Base
  belongs_to :role
  belongs_to :user
end
class Role < ActiveRecord::Base
  has_many :permissions
  has_many :users, :through => :permissions
end

I want to be able to look at permissions in a few different ways (and roles and users for that matter). Now, I can modify my PermissionsController to set the object that will be used for all find calls. By default, the PermissionsController will use Permission.find, but if I swap the Permission object for @finder_object, I can set that @finder_object before_filter. See below:

class PermissionsController < ActiveRecord::Base
  before_filter :determine_finder_object

  def index
    @permissions = @finder_object.find(:all)
    # respond to stuff, though you might want to use different templates
    # based on the finder object
  end

  def determine_finder_object
    if params[:user_id]
      @finder_object = User.find(params[:user_id]).permissions
    elsif params[:role_id]
      @finder_object = Role.find(params[:role_id]).permissions
    else
      @finder_object = Permission
    end
  end
end

Now, is this the best practice? I don't know, however, it feels like its something to explore. One noticeable caveat is that if you are using a REST-based controller, you will want to look at all of your Object.new calls and think about replacing them with @finder_object.build.

As an aside, the above controller would probably end up with several chunks of code that were:

  if params[:user_id]
    #do something for users
  elsif params[:role_id]
    #do something for roles
  else
    #do something in the generic
  end

What can I do to encapsulate this particular logic? I dug up the code from the :respond_to controller instance method, and created a construct for:

determine do |d| 
 d.user_id { #do something for users}
 d.role_id { #do something for roles}
end

I'm not satisfied with it, but am wondering what I might be able to do.