tags.rb

create_table "taggings", :force => true do |t|
  t.integer "tag_id"
  t.string  "taggable_type", :default => ""
  t.integer "taggable_id"
end

class Post
  has_many :taggins, :polymorphic => true

  def tags
    tag_used_in_scope = Tagging.arel_table[:taggable_id].in(self.select(:id).arel)
    taggable_has_correct_type = Tagging.arel_table[:taggable_type].eq(self.klass.to_s)
    Tag.where(tag_used_in_scope.and(taggable_has_correct_type))
  end
end

blog.posts.tags => # wszystkie tagi, ktore wystepuja w postach