Why isn't this Ruby on Rails / Paperclip form showing up?

I'm getting started with Paperclip and can't figure out something: why isn't this form showing up?

in sources_controller.rb

class SourcesController < ApplicationController

  def new
    @source = Source.new
    @title = "New Source"
  end

end

in new.html.erb

New Source Form

<%= debug(@source) %>

<% form_for(@source, :html => { :multipart => true }) do |f| %>
  <div class="field">
    <%= f.label :name %><br />
    <%= f.text_field :name %>
  </div>
  <div class="field">
    <%= f.label :attachment %><br />
    <%= f.file_field :attachment %>
  </div>
  <div class="field">
    <%= f.submit 'Create' %>
  </div>
<% end %>

What gets rendered in a browser:

New Source Form

--- !ruby/object:Source 
aggregation_cache: {}

association_cache: {}

attributes: 
  attachment_file_name: 
  name: 
  attachment_file_size: 
  attachment_content_type: 
  created_at: 
  parent_asset_id: 
  updated_at: 
  is_directory: 
  id: 
  user_id: 
  attachment_updated_at: 
  access_token: 
attributes_cache: 
  created_at: 
  updated_a开发者_开发问答t: 
  attachment_updated_at: 
changed_attributes: {}

destroyed: false
marked_for_destruction: false
new_record: true
previously_changed: {}

readonly: false

(When the debug isn't present, only "New Source Form" shows up.

---

Wow, I can't believe this is happening.

The line <% form_for(@source, :html => { :multipart => true }) do |f| %> should be <%= form_for(@source, :html => { :multipart => true }) do |f| %> (with an equals sign).

This bug took me forEVER to figure out.

Why doesn't RoR give me a warning or something?! Can someone give me some tips to help prevent this from happening again?

---

This is no bug, since the form is something you actually want to have displayed. Glad, that you found it out on your own ;-)