Question

I am building a Rails Engine. I defined a controller like this,

module A::B::C::D::E
  extend ActiveSupport::Concern

  # module stuff ... 
end

class ExamplesController < ApplicationController
  include A::B::C::D::E

  # controller stuff ...
end

When I start the Rails console, rails console, I get the following error,

uninitialized constant A::B (NameError)

Why do I get this error?

Answer 1

Because the module does not exist at the point you are using it.
Since A is not defined, Ruby doesn’t know what it is.

Note that the :: is a scope resolution operator used for look up, not defining a namespace-like hierarchy.

It would work if you defined A first:

module A
end

Then B:

module A::B
end

Then C:

module A::B::C
end

And so on.

Of course you could also do this:

module A
  module B
    module C
      module D
        module E
        end
      end
    end
  end
end