title: Ruby on rails errors undefined method `map' for nil:NilClass
body: <p>Below is code extract from my file /home/divya/climb/project1/app/views/cities/new.html.erb where line #5 raised this error:</p>
  
  <blockquote>
    <p>undefined method `map' for nil:NilClass</p>
  </blockquote>
  
  <p>Extracted source (around line #5):</p>
  
  <pre><code>2: &lt;%= form_for(@city) do |f| %&gt;
  3: &lt;%= f.label :country_id %&gt;&lt;br /&gt;
  4: 
  5: &lt;%= collection_select(:city, :country_id, @countries, :id, :country_name, {:prompt =&gt; false}) %&gt;
  6: &lt;%= render 'form' %&gt;
  7: 
  8: &lt;%= link_to 'Back', cities_path %&gt;
  
  Rails.root: /home/divya/climb/project
  </code></pre>