class AbstractTopLevelController < ApplicationController
  class_inheritable_accessor :model

  def index
    model.classify.constantize.all
  end
end

class AbstractNestedController < ApplicationController
  class_inheritable_accessor :model
  class_inheritable_accessor :parent

  def index
    parent.classify.constantize.find(params["#{parent}_id".to_sym].send(model).all
  end
end

class FooController < AbstractTopLevelController
  self.model = "foo"
end

class NestedFooController < AbstractNestedController
  self.model = "foo"
  self.parent = "bar"
end