あるモデルを複数ファイに記述する

# config/initializers/concerns.rb
class << ActiveRecord::Base
  def concerned_with(*concerns)
    concerns.each do |concern|
      require_dependency "#{name.underscore}/#{concern}"
    end
  end
end

# app/models/product.rb
class Product < ActiveRecord::Base
  concerned_with :separate
end

# app/models/product/separate.rb
class Product
  def separated_func
    ...
  end
end