Creating view for adding nested resource in Ruby on Rails app

I'm making a RoR app that has 3 resources, that are nested in this order: Projects -> Feeds -> XML_Fields. Projects has_many Feeds, Feeds has_many XML_Fields. My problem is that I am getting stuck on adding to the Show Feeds view the ability to add a new XML_Field. My code is below:

<h2>Add an XML field:</h2>
<%= form_for ([@feed, @feed.xml_fields.build]) do |f| %>
  <div class="field">
  <%= f.text_area :tag %>
  </div>
  <div class="actions">
  <%= f.submit %>
  </div>开发者_如何转开发
<% end %>

Executing this returns a NoMethodError: undefined method 'tag'. Tag is the name of the column in the xml_fields table I created. My xml_fields_controller has this create method:

def create
@xml_field = Feed.find(params[:feed_id]).xml_fields.build(params[:xml_field])

respond_to do |format|
  if @xml_field.save
    format.html { redirect_to( :back, :notice => 'XML Field was successfully created.') }
    format.xml  { render :xml => @xml_field, :status => :created, :location => [@xml_field.feed, @xml_field] }
  else
    format.html { render :action => "new" }
    format.xml  { render :xml => @xml_field.errors, :status => :unprocessable_entity }
  end
end

end

And my routing file looks like this:

resources :projects do
  resources :feeds
end

resources :feeds do
  resources :xml_fields
end

Any suggestions to make this NoMethodError go away? Thanks.

model XmlField doesn't have tag field in database ;)