module Uuids

  # Creates required `#uuids` attribute of the ActiveRecord model.
  module Base
    extend ActiveSupport::Concern

    # Methods added to the ActiveRecord model.
    module ClassMethods

      private

      # Declares:
      #
      # +uuids+:: association attribute;
      # +uuid+:: method (string);
      # <tt>uuid</tt>:: relation scope.
      #
      # @example
      #     class City < ActiveRecord::Base
      #       has_uuids
      #     end
      #
      #     city = City.create!
      #
      #     city.uuids.map(&:value)
      #     # => 51f50391-fcd2-4f69-aab7-6ef31b29c379
      #
      #     city.uuid
      #     # => 51f50391-fcd2-4f69-aab7-6ef31b29c379
      #
      #     City.by_uuid("51f50391-fcd2-4f69-aab7-6ef31b29c379").to_a == [city]
      #     # => true
      #
      def has_uuids
        define_uuids
        define_uuid_getter
        define_scope
      end

      # Defines the +uuids+ association attribute;
      def define_uuids
        has_many :uuids, class_name: "Uuids::Uuid", as: :record, validate: false
        before_create  :add_default_uuid
        before_destroy :prevent_destruction
        validates :uuids, presence: true, on: :update
      end

      # Defines the +uuid+ method
      def define_uuid_getter
        class_eval "def uuid; uuids.map(&:value).sort.first; end"
      end

      # Defines the <tt>uuid</tt> relation scope.
      def define_scope
        scope(
          :by_uuid,
          ->(value) { joins(:uuids).where(uuids_uuids: { value: value }).uniq }
        )
      end
    end

    private

    # Prevents the module usage outside an ActiveRecord model.
    def self.included(klass)
      unless klass.ancestors.include? ActiveRecord::Base
        fail TypeError.new("#{ klass.name } isn't an ActiveRecord model.")
      end
    end

    # Creates the uuids by default preventing a record from being ureferrable
    def add_default_uuid
      uuids.present? || uuids.new
    end

    # Prevents destruction of a record before its uuids assigned to another one
    def prevent_destruction
      return true if uuids.blank?
      errors.add :base, :uuids_present
      false
    end
  end
end