Dynamic namespaced controllers w/ fallback in Rails - ruby-on-rails

Question

I have a somewhat bizarre requirement for a new Rails application. I need to build an application in which all routes are defined in multiple namespaces (let me explain). I want to have an application in which school subjects (math, english, etc) are the namespaces:
%w[math english].each do |subject|
namespace subject.to_sym do
resources :students
end
end
This is great and it works but it requires me to create a namespaced StudentsController for each subject which means if I add a new subject then I need to create a new controller.
What I would like is to create a Base::StudentsController and if, let's say the Math::StudentsController exists then it will be used and if it doesn't exist, then we can dynamically create this controller and inherit from Base::StudentsController.
Is this something that is possible? If so then how would I go about implementing this?

Answer 1

With routes defined this way:
%w[math english].each do |subject|
scope "/#{subject}" do
begin
"#{subject.camelcase}::StudentsController".constantize
resources :students, controller: "#{subject}::students", only: :index
rescue
resources :students, controller: "base::students", only: :index
end
end
end
rake routes outputs:
students GET /math/students(.:format) base::students#index
GET /english/students(.:format) english::students#index
if english/students_controller.rb is present and math/students_controller. is absent.

Answer 2

To restate your requirements:
Minimal declarations per subject/resource pair
Use dedicated controller (Math::StudentsController) if it exists, otherwise use base controller (StudentsController)
Rails expects each route to have a dedicated controller, and doesn't really have a good way to support the second requirement. So, this is how I would do it:
Dynamicroutes::Application.routes.draw do
SUBJECTS = [ "math", "english", "chemistry" ]
RESOURCES = [ "assignments", "students" ]
class DedicatedSubjectResourceControllerConstraint
def initialize(subject, resource)
#subject = subject
#resource = resource
end
def matches?(request)
begin
defined?("#{#subject.capitalize}::#{#resource.capitalize}")
return true
rescue NameError
Rails.logger.debug "No such class: #{#subject.capitalize}::#{#resource.capitalize}"
return false
end
end
end
class ValidSubjectConstraint
def matches?(request)
return SUBJECTS.include?(request.path_parameters[:subject])
end
end
SUBJECTS.each do |subject|
RESOURCES.each do |resource|
namespace subject, :constraints => DedicatedSubjectResourceControllerConstraint.new(subject, resource) do
resources resource
end
end
end
RESOURCES.each do |resource|
scope "/:subject", :constraints => ValidSubjectConstraint.new do
resources resource
end
end
end

Answer 3

This sounds like a use for const_missing. If what you want to do is
to create a Base::StudentsController
and if, let's say the Math::StudentsController exists
then it will be used
and if it doesn't exist, then we can dynamically create this controller and inherit from Base::StudentsController
You can achieve that by adding dynamic constant lookup (const_missing) and dynamic constant definition with inheritance (Object.const_set).
I imagine something like this; with a few tweaks and more rigorous checking, would work:
# initializers/dynamic_controllers.rb
class ActionDispatch::Routing::RouteSet
SUBJECTS = [ "math", "english", "chemistry" ]
def const_missing(name, *args, &block)
if SUBJECTS.any?{ |subject| name.include? subject.uppercase }
Object.const_set name, Class.new(Base::StudentsController)
else
super
end
end
end
That'll add dynamic constant lookups to ActionDispatch::Routing::RouteSet, from which Dynamicroutes::Application.routes inherits, so undefined constants in Dynamicroutes::Application.routes.draw will generate the corresponding classes subclassed from Base::StudentsController.

Answer 4

I believe this will do it:
%w[math english].each do |subject|
namespace subject.to_sym do
resources :students
end
end
match ':subject/students(/:action(/:id))' => 'base/students'
With these combined routes, /math/students goes to the Math::StudentsController, /english/students/ goes to the English::StudentsController, and all other subjects (e.g. /physics/students and /cs/students) go to the Base::StudentsController.
Which I think is exactly what you want and only adds one line of code to your original solution.

Answer 5

All the routing helpers like resources, scope, etc are just functions inside your application's routes. You could just define a custom function as follows:
YourApplication.routes.draw do
# Let's define a custom method that you're going to use for your specific needs
def resources_with_fallback(*args, &block)
target_module = #scope[:module].camelize.constantize
target_controller = "#{args.first.to_s}_controller".camelize
fallback_controller = args.last.delete(:fallback).to_s.camelize.constantize
# Create the target controller class
# using fallback_controller as the superclass
# if it doesn't exist
unless target_module.const_defined?(target_controller)
target_module.const_set target_controller, Class.new(fallback_controller)
end
# Call original resources method
resources *args, &block
end
# Now go ahead and define your routes!
namespace "test" do
namespace "new" do
# Use our custom_resources function and pass a fallback parameter
custom_resources :photos, :fallback => 'base/some_controller'
end
end
end
I tested this in Rails 3.2, but it should equally work well in all 3.x versions.
I included no null checks or begin/rescue blocks anywhere. Since you're going to use this custom function only when required, I'm assuming that you will pass the correct and necessary parameters. If say you passed a fallback controller that doesn't exist, I'd rather that the routes parsing fail with an exception, rather than trying to handle it.
Edit: Typo in function arguments
Edit 2: Forgot &block in function arguments
Edit 3: Add "_controller" to the target_controller variable

Answer 6

I ended up writing some custom logic into ActionDispatch::Routing::RouteSet::Dispatcher.controller_reference. I attempt to look up all of the constants required for the given controller and create them if they're missing. This code is FAR from perfect so please feel free to edit w/ improvements.
class ActionDispatch::Routing::RouteSet::Dispatcher
private
def controller_reference(controller_param)
const_name = #controller_class_names[controller_param] ||= "#{controller_param.camelize}Controller"
obj = Object
const_name.split('::').each do |cn|
begin
obj = obj.const_get(cn)
rescue
if obj == Object
obj = obj.const_set(cn, Class.new(ApplicationController))
else
puts "Creating #{obj}::#{cn} based on Generic::#{cn}"
obj = obj.const_set(cn, Class.new("Generic::#{cn}".constantize))
end
end
end
ActiveSupport::Dependencies.constantize(const_name)
end
end