title: What is the proper use of the check_box form helper?
body: <p>I have two models - <code>Client</code> &amp; <code>Topic</code>, with a HABTM relationship between them.</p>
  
  <p>I am trying to generate a series of checkboxes of the topics, on the Client <code>form</code> partial.</p>
  
  <p>This is what I am doing:</p>
  
  <pre><code>&lt;% Topic.all.each do |topic| %&gt;
    &lt;% checked = @client.topics.include?(topic) %&gt;
      &lt;%= f.label(:name, topic.name) %&gt; &lt;%= f.check_box @topics, topic.id %&gt;
  &lt;% end %&gt;
  </code></pre>
  
  <p>This is the error I get:</p>
  
  <pre><code>undefined method `merge' for 1:Fixnum
  </code></pre>
  
  <p>I know one solution is to use <code>check_box_tag</code>, but that forces me to do the record updating of the associations manually.</p>
  
  <p>So I would rather use the form_helper for the checkbox tag. <a href="http://api.rubyonrails.org/classes/ActionView/Helpers/FormHelper.html#method-i-check_box" rel="nofollow">The docs</a> are a bit confusing to me.</p>
  
  <p>How can I get this to work with <code>f.check_box</code>.</p>
  
  <p>Thanks.</p>