mongrel won't stay running

I think the problem in my previous post was caused by not restarting
mongrel.  Now that I have restarted mongrel, it won't stay running.

 $ less mongrel.8011.log
** Daemonized, any open files are closed.  Look at
tmp/pids/mongrel.8011.pid and log/mongrel.8011.log for info.
** Starting Mongrel listening at 127.0.0.1:8011
** Starting Rails with production environment...
/usr/websites/papers.pgcon.org/pentabarf/rails/app/models/release/conference.rb:5:
undefined method `fk_helper_single' for Release::Conference:Class
(NoMethodError)
        from
/usr/local/lib/ruby/site_ruby/1.8/rubygems/custom_require.rb:27:in
`gem_original_require'
        from
/usr/local/lib/ruby/site_ruby/1.8/rubygems/custom_require.rb:27:in
`require'
        from
/usr/local/lib/ruby/gems/1.8/gems/activesupport-2.2.2/lib/active_support/dependencies.rb:155:in
`require'
        from
/usr/local/lib/ruby/gems/1.8/gems/activesupport-2.2.2/lib/active_support/dependencies.rb:262:in
`require_or_load'
        from
/usr/local/lib/ruby/gems/1.8/gems/activesupport-2.2.2/lib/active_support/dependencies.rb:221:in
`depend_on'
        from
/usr/local/lib/ruby/gems/1.8/gems/activesupport-2.2.2/lib/active_support/dependencies.rb:133:in
`require_dependency'
        from