module DataMapper
  module Is
    module Commentable
      def is_commentable
        include DataMapper::Is::Commentable::InstanceMethods
        has n, :comments, :child_key => [:object_id], :object_type => "#{self}"
      end

      module InstanceMethods
        def build_comment(attrs = {})
          Comment.new(attrs.merge(:object_id => self.id, :object_type => "#{self.class}"))
        end

        def create_comment(attrs = {})
          Comment.create(attrs.merge(:object_id => self.id, :object_type => "#{self.class}"))
        end

        def ordered_comments
          Comment.all(:object_id => self.id, :object_type => self.class, :order => [:created_at.desc])
        end
      end
    end
  end
end

# Include the plugin in Resource
module DataMapper
  module Resource
    module ClassMethods
      include DataMapper::Is::Commentable
    end # module ClassMethods
  end # module Resource
end # module DataMapper


class Comment
  include DataMapper::Resource

  property :id,                 Integer, :serial => true
  property :body,               Text
  property :created_at,         DateTime
  property :user_id,            Integer
  property :object_id,          Integer
  property :object_type,        String

  belongs_to :user

  validates_present :body
end