respond_to do |format|
  format.html # index.html.erb
  format.xml  { render :xml => @posts }
end

The problem is that the generated xml is very verbose. I spent some time googling and found plenty of information, but it was scattered and didn’t quite show the whole picture.

The rails api for Builder::XmlMarkup uses both an ‘xm’ and ‘xml’ object to show how to work with xml, but only the ‘xml’ object seems to work. It also doesn’t clearly show how to setup the controller or that the code should go in an xml.builder file. Similarly here, the xml object is used in the view correctly, but also fails mention anything about the controller or type of file the code should go in.

Over at xml.com, the tutorial creates an @xml instance variable of type Builder::XmlMarkup.new and then uses that object in an .rxml view (I know, its a little dated).

I was left with a bunch of puzzle pieces that didn’t quite fit together. Eventually, I figured out what needed to be done, and this is how…

First, in the controller, remove the other bits in the format.xml line so it reads

respond_to do |format|
  format.html # index.html.erb
  format.xml # index.xml.builder
end

Second, you need to create an xml.builder file, in this case, index.xml.builder. Inside the xml.builder view, you have access to an ‘xml’ object that is used to generate the xml. The xml for my sample @posts object can then look something like…

xml.instruct!
xml.posts do
  @posts.each do |post|
    xml.post do
      xml.title post.title
      xml.body post.body
      xml.published_at post.published_at
      xml.comments do
        post.comments.each do |comment|
          xml.comment do
            xml.body comment.body
          end
        end
      end
    end
  end
end

Now, when browsing to posts.xml, you’ll now see nicely formatted xml, without all the extra cruft added when using the default rails xml generation utility. I hope that saves you some time.