Accessing class constant from within a module

Question

I need to access an ActiveRecord constant from an included module in a Rails app.

class User < ApplicationRecord
  include ModOne
  
  OPTIONS = {a: 1}.freeze

  ...
end

Module ModOne
  extend ActiveSupport::Concern

  included do
    do_semething(self::OPTIONS)
  end

  class_methods do
   def do_something(opts)
     ...
   end
  end
end

But I get

NameError: uninitialized constant User (call 'User.connection' to establish a connection)::OPTIONS Did you mean? User::OPTIONS

What am I missing here?

I have also tried to replace self with base_class and event User but I get the same error.

Answer 1

It's simply a matter of order. You have to define the constant first. Ruby classes are really just a block of code and run top down and when you call include you're also calling the #included method on the module.

But a better approach if you want the functionality provided by a module to be customizable is to just write a so called macro method instead of hinging everything on the Module#included hook:

# You can obscure this with some syntactic sugar from ActiveSupport::Concern 
# if it makes you happy.
module Awesomeizer
  def foo
    self.class.awesomeize_options[:foo]
  end

  module ClassMethods
    def awesomeize_options
      @awesomeize_options ||= defaults
    end

    def awesomeize(**options) 
       awesomeize_options.merge!(options)
    end 

    def defaults
      {}
    end
  end

  def self.included(base)
    base.extend(ClassMethods)
  end
end

class Thing
  include Awesomeizer
  awesomeize(foo: :bar)
end

This pattern can be found everywhere in Ruby and is great way to get around the fact that Module#include doesn't allow you to pass any additional arguments.

In this example the awesomeize method just stores the options as a class instance variable.

One of the strongest reasons why this is preferable is that it lets you define a signature for the interface between the module and it's consumers instead of just making assumptions. Some gems like Devise even use this method to include it's submodules.