xml.instruct!
xml.posts(:type => "array") do
  @posts.each do |post|
    xml.post do
      xml.id(post.id)
      xml.title(post.title)
      xml.fulltext(post.fulltext)
      xml.created_at(post.created_at, :type => "datetime")
      xml.updated_at(post.updated_at, :type => "datetime")
    end
  end
end