Q:

What is causing this NoMethodError in Ruby on Rails?

This is the code I have at the end a the create method in 
Notes controller
  def create
    @note = Note.new(params[:note])
    @note.user_id = current_user.id
    respond_to do |format|
      if @note.save
        flash[:success] = 'Note was created.'
        # @note.note_type is "Temp" at this point
        if @note.note_type == "Temp"
          p "passed first test"
          redirect_to :controller => :categories, :action => :add_category, :note_id => @note.id, :category => 'temp category'
        else
          format.html { redirect_to(@note) }
          format.xml  { render :xml => @note, :status => :created, :location => @note }
        end
      else
        format.html { render :action => "new" }
        format.xml  { render :xml => @note.errors, :status => :unprocessable_entity }
      end
    end
  end

Categories controller
def add_category
    p "in add_category"
end

When it runs, I get the following error. Line 80, where the trace begins, is the respond_to do |format| line:
NoMethodError in NotesController#create 
undefined method `call' for nil:NilClass
/usr/local/lib/ruby/gems/1.8/gems/activesupport-2.3.5/lib/active_support/whiny_nil.rb:52:in `method_missing'
/usr/local/lib/ruby/gems/1.8/gems/actionpack-2.3.5/lib/action_controller/mime_responds.rb:175:in `respond'
/usr/local/lib/ruby/gems/1.8/gems/actionpack-2.3.5/lib/action_controller/mime_responds.rb:173:in `each'
/usr/local/lib/ruby/gems/1.8/gems/actionpack-2.3.5/lib/action_controller/mime_responds.rb:173:in `respond'
/usr/local/lib/ruby/gems/1.8/gems/actionpack-2.3.5/lib/action_controller/mime_responds.rb:107:in `respond_to'
/Users/benhartney/rails_projects/note_taker/app/controllers/notes_controller.rb:80:in `create'

I've got no idea how to start debugging this error. The console shows the output line "passed first test" from the create action in the Notes controller, but not the "in add_category" output line from the add_category action in the Categories controller. I'm using Rails 2.3.5. Any help would be greatly appreciated. Thanks for reading.

A:

The respond_to method requires you to specify at least one format on the Responder object passed into its block.  In the code-path creating your exception, you just call redirect_to, the responder MIME-type preference list is empty, and Rails ends up trying to call the call method on the (non-existent) block for the (non-existent) first type preference.

