Rails chooses wrong class from wrong namespace

TL;DR Load order problem. Always use namespaces for classes that have the same base name, i.e. don’t use Category and Admin::Category (Admin is also a class), but Front::Category and Admin::Category. Or explicitly set load order if you can. Or change Admin from class to a module.

Did you encountered something like this?

/home/hrdina/lightway/test/controllers/merchant/adverts/images_controller_test.rb:3: warning: toplevel constant Adverts referenced by Merchant::Adverts
/home/hrdina/lightway/test/controllers/merchant/adverts/images_controller_test.rb:5: warning: toplevel constant Adverts referenced by Merchant::Adverts

and tests are failing, right? You say that variables aren’t set. Ok, (after a few hours with messing around) some debugging:

test "gets index" do
  get :index, advert_id: @merchant_advert.id
  assert_response :success

  puts assigns.inspect  # <-- OUR DEBUG CODE
  assert_not_nil assigns(:merchant_advert_images)
end

It prints instance variables

...ode=:sk, @errors=#<ActiveModel::Errors:0x007fdb412f5c28 @base=#<Support::Language:0x007fdb414161c0 ...>, @messages={:code=>[]}>>,
"advert"=>#<Advert id: 3, merchant_id: 3, category_id: 6, ....>,
"advert_images"=>#<ActiveRecord::Association...

Hmm, only advert and advert_images are set, not merchant_advert and merchant_advert_images as they should be - it calls other controller, AdvertsController! Here is some explanation which I copied somewhere on stackoverflow.org:

my run in irb:

irb(main):001:0> Document = 'hello'
=> "hello"
irb(main):002:0> 
irb(main):003:0* class Question
irb(main):004:1> end
=> nil
irb(main):005:0> 
irb(main):006:0* class Animal
irb(main):007:1>   puts Question::Document
irb(main):008:1> end
(irb):7: warning: toplevel constant Document referenced by Question::Document
hello
=> nil
irb(main):009:0> 
irb(main):010:0* class Question
irb(main):011:1>   class Document
irb(main):012:2>   end
irb(main):013:1> end
=> nil

and the next explanation from Andrew White on https://groups.google.com/forum/#!topic/rubyonrails-core/urXofuENLss:

“Is Admin a class or a module? If it’s a class it’s due to the fact that const_missing searches for constants in ancestors which includes Object when Admin is a class. Since SomeController is defined already in Object it will return that constant along with the warning message. If Admin is a module then the ancestor list is just itself.

A couple of ways to workaround the problem, either make Admin a module if possible or nest the public-facing controllers under a different namespace, e.g. Admin::SomeController and Front::SomeController.”

Conclusion

I think the best way to handle this, is to enforce modules by design. It will require to change some application logic and routes, but it is worth of it.