Question

Why do I get error like this?

undefined method `community_topics_path' for #<#<Class:0x00000009d79098>:0x00000009d70a38>

Extracted source (around line #1):

1: <%= form_for @community_topic, :html => { :class => 'form-horizontal' } do |f| %>
2:   <div class="control-group">
3:     <%= f.label :community_id, :class => 'control-label' %>
4:     <div class="controls">

rake routes shows I use ‘to_param’ for community’s id but I haven’t defined :community_id at all in my routes.rb. I wonder why rake routes shows this :community_id. Is that probably because I use ‘to_param’ for my community model?? That’s why it automatically detect that and replace :id to :community_id?

new_community_topic GET    /communities/:community_id/topic/new(.:format)      community_topics#new

routes.rb

resources :communities do
    resources :community_topics, :path => "topic", :as => :'topic'
end

views/communities/_form.html.erb

<%= form_for @community_topic, :html => { :class => 'form-horizontal' } do |f| %>

.......

  <div class="form-actions">
    <%= f.submit nil, :class => 'btn btn-primary' %>
    <%= link_to t('.cancel', :default => t("helpers.links.cancel")),
                community_topic_path, :class => 'btn' %>
  </div>
<% end %>

Updated!!!

**rake routes | grep community_topic

   community_topic_index GET    /communities/:community_id/topic(.:format)          community_topics#index
                         POST   /communities/:community_id/topic(.:format)          community_topics#create
     new_community_topic GET    /communities/:community_id/topic/new(.:format)      community_topics#new
    edit_community_topic GET    /communities/:community_id/topic/:id/edit(.:format) community_topics#edit
         community_topic GET    /communities/:community_id/topic/:id(.:format)      community_topics#show
                         PUT    /communities/:community_id/topic/:id(.:format)      community_topics#update
                         DELETE /communities/:community_id/topic/:id(.:format)      community_topics#destroy

Answer 1

As you are using nested routing then you have to pass and community into form_for:

<%= form_for [@community, @community_topic], :html => { :class => 'form-horizontal' } do |f| %>

upd: Or @community_topic.community in case you ain’t set @community.

<%= form_for [@community_topic.community, @community_topic], :html => { :class => 'form-horizontal' } do |f| %>

You can watch this episode of RailsCasts to fully understand nested resources. Episode use Rails 2 for examples but you should understand the conception.