NoMethodError in Feeds#rss

Showing app/views/feeds/rss.rxml where line #40 raised: 

undefined method `post_to_html' for #<#<Class:0xb7205334>:0xb72052f8>

Extracted source (around line #40): 

37: 
38:         post.tags.each { |tag| builder.category tag }
39: 
40:         content   = REXML::Document.new(post_to_html(post))
41: 
42:         make_relative_urls_fixed(content, base_url)
43:

RAILS_ROOT: /var/www/rikkus.info/config/..

Application Trace | Framework Trace | Full Trace 
#{RAILS_ROOT}/app/views/feeds/rss.rxml:40:in `_run_rxml_47app47views47feeds47rss46rxml'
#{RAILS_ROOT}/app/views/feeds/rss.rxml:29:in `_run_rxml_47app47views47feeds47rss46rxml'
#{RAILS_ROOT}/app/views/feeds/rss.rxml:27:in `each'
#{RAILS_ROOT}/app/views/feeds/rss.rxml:27:in `_run_rxml_47app47views47feeds47rss46rxml'
#{RAILS_ROOT}/app/views/feeds/rss.rxml:13:in `_run_rxml_47app47views47feeds47rss46rxml'
#{RAILS_ROOT}/app/views/feeds/rss.rxml:11:in `_run_rxml_47app47views47feeds47rss46rxml'
#{RAILS_ROOT}/app/controllers/feeds_controller.rb:15:in `rss'
/usr/bin/mongrel_rails:16:in `load'
/usr/bin/mongrel_rails:16

Request

Parameters: None

Show session dump

Response

Headers: {"cookie"=>[], "Content-Type"=>"application/xml", "Cache-Control"=>"no-cache"}