data_migrator.rb

require 'singleton'
require 'forwardable'

module TradeTariffBackend
  class DataMigration
    include Singleton
    extend SingleForwardable

    module BlockAccessor
      extend ActiveSupport::Concern

      module ClassMethods
        def block_accessor(*names)
          names.each do |name|
            define_method(name) do |&block|
              if block
                instance_variable_set(:"@#{name}", block)
              else
                # lazy call
                instance_variable_get(:"@#{name}") #.call
              end
            end
          end
        end
      end
    end

    class Runner
      include BlockAccessor

      block_accessor :met, :meet

      def initialize(&block)
        instance_eval &block
      end
    end

    def_delegators :instance, :applied?
    delegate :up, :down, to: self.class

    def up_runner
      @up_runner ||= Runner.new(&up)
    end

    def down_runner
      @down_runner ||= Runner.new(&down)
    end

    def applied?
      up_runner.met #.call
    end

    def can_rollback?
      down_runner.met #.call
    end

    def created_at
      # parse from filename or created_at
      # or default to NullDate?
    end

    class << self
      include BlockAccessor

      block_accessor :up, :down
    end

    def self.[](migration_name)
      # DataMigrator.migrations...?
    end
  end
  
  class Dependency
    attr_reader :migrations

    def self.[](*migrations)
       new(migrations)
    end
    
    def new(migrations)
      @migrations = migrations
    end
    
    def met?
      migrations.all?(&:met?)
    end
  end

  class ExampleMigration < DataMigration
    up do
      met {
        Dependency[AnotherMigration, SomeOtherMigration].met? && Chief::Mfcm.where(cmdty_code: '00000000').none?
      }
      meet {}
    end

    down do
      met  { false }
      meet {}
    end
  end

  class DataMigrator
    class Presenter
      def self.present(migrations, presenter = ConsolePresenter)
        migrations.sort_by(&:created_at).each {|migration|
          presenter.present(migration)
        }
      end
    end

    class ConsolePreseter < Presenter
      def self.present(migration)
        puts "[ #{migration.state} ] #{migration.name}"
      end
    end

    class << self
      def migrations
        DataMigration.descendants.map(&:instance)
      end
    end
  end
end