Untitled

module Versionable
  extend ActiveSupport::Concern

  BASE_VERSION = 1
  VERSION_PROPERTY_NAME = 'schema_version'

  included do
    property VERSION_PROPERTY_NAME.to_sym, Integer, numericality: { greater_than_or_equal_to: 0 }, default: lambda { current_schema_version }
    extend (embeddable? ? EmbeddedDocument : Document)
  end

  module ClassMethods
    attr_accessor :migrate_schema_block
    def migrate_schema(&block)
      self.migrate_schema_block = block
    end

    def current_schema_version(version = nil)
      if version
        @_current_schema_version = version
      else
        @_current_schema_version or raise "current_schema_version is not set"
      end
    end

  private

    def instantiate(robject_or_hash, data)
      super(robject_or_hash).tap do |document|
        if document.schema_version < current_schema_version && migrate_schema_block
          document.instance_exec(data, &migrate_schema_block)
        elsif document.schema_version > current_schema_version
          raise "Cannot migrate #{self} from version #{document.schema_version} to #{current_schema_version}"
        end
      end
    end
  end

  module Document
    private
    def instantiate(robject)
      robject.data[VERSION_PROPERTY_NAME] ||= BASE_VERSION
      super(robject, robject.data)
    end
  end

  module EmbeddedDocument
    def instantiate(hash)
      hash[VERSION_PROPERTY_NAME] ||= BASE_VERSION
      super(hash, hash)
    end
  end
end