NoMethodError in Feeds#atom

Showing app/views/feeds/atom.rxml where line #43 raised: 

undefined method `post_to_html' for #<#<Class:0xb6e9fdc0>:0xb6e9fd84>

Extracted source (around line #43): 

40:         builder.category(:term => tag, :label => tag)
41:       }
42: 
43:       content   = REXML::Document.new(post_to_html(post))
44: 
45:       make_relative_urls_fixed(content, base_url)
46:

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

Application Trace | Framework Trace | Full Trace 
#{RAILS_ROOT}/app/views/feeds/atom.rxml:43:in `_run_rxml_47app47views47feeds47atom46rxml'
#{RAILS_ROOT}/app/views/feeds/atom.rxml:24:in `_run_rxml_47app47views47feeds47atom46rxml'
#{RAILS_ROOT}/app/views/feeds/atom.rxml:22:in `each'
#{RAILS_ROOT}/app/views/feeds/atom.rxml:22:in `_run_rxml_47app47views47feeds47atom46rxml'
#{RAILS_ROOT}/app/views/feeds/atom.rxml:11:in `_run_rxml_47app47views47feeds47atom46rxml'
#{RAILS_ROOT}/app/controllers/feeds_controller.rb:21:in `atom'
/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"}