Example of class @instance_var that used to be a @@class_var

categorizable.rb

module Category::Categorizable

  extend ActiveSupport::Concern

  included do

    def self.categorizable_through(table)
      has_many table, dependent: :destroy
      @categorizable_join_model = Kernel.const_get(table.to_s.singularize.classify)
    end

    def categories
      Category::CategoriesRelation.new(
        self,
        self.class.instance_variable_get(:@categorizable_join_model)
      )
    end

  end

end

categorizable_old.rb

module Category::Categorizable

  extend ActiveSupport::Concern

  included do

    def self.categorizable_through(table)
      @@categorizable_join_model = Kernel.const_get(table.to_s.singularize.classify)
    end

    def categories
      Category::CategoriesRelation.new(self, @@categorizable_join_model)
    end

  end

end

usage.rb

class Post < ActiveRecord::Base

  include Category::Categorizable
  categorizable_through :categories_posts

end