gistfile1.rb

# domains/show.html.haml
%h2 Keywords
= link_to "New keyword", new_domain_keyword_path(:domain_id => @domain.id)

# keywords/new.html.haml
= form_for([@domain, @keyword]) do |f|
  = f.label :term
  = f.text :term

# controllers/keywords_controller.rb
class KeywordsController < ApplicationController
  def new
    @domain = Domain.find(params[:domain_id])
    @keyword = @domain.keywords.new
  end
end

# Scheme
  create_table "domains", :force => true do |t|
    t.string   "name"
    t.string   "url"
    t.datetime "created_at"
    t.datetime "updated_at"
  end

  create_table "domains_keywords", :force => true do |t|
    t.integer "domain_id"
    t.integer "ticket_id"
  end

  create_table "keywords", :force => true do |t|
    t.integer  "rank"
    t.string   "term"
    t.integer  "domain_id"
    t.datetime "created_at"
    t.datetime "updated_at"
  end