classic_approach.rb

module Imageable
  def self.included(model_class)
    model_class.has_many :images, polymorphic: true
    model_class.extend ClassMethods
  end

  module ClassMethods
    def has_one_image(*names)
      names.each do |image_kind|
        define_method image_kind do
          images.find { |image| image.kind == image_kind }
        end
      end
    end
  end
end

class Project
  include Imageable

  has_one_image :main_image, :secondary_image
end