title: How to express a NOT IN query with ActiveRecord/Rails?
body: <p>I'm hoping there is a easy solution that doesn't involve <code>find_by_sql</code>, if not then I guess that will have to work.</p>
  <p>I found <a href="http://trevorturk.com/2007/03/19/active-record-and-the-in-clause/" rel="noreferrer">this article</a> which references this:</p>
  <pre><code>Topic.find(:all, :conditions =&gt; { :forum_id =&gt; @forums.map(&amp;:id) })
  </code></pre>
  <p>which is the same as</p>
  <pre><code>SELECT * FROM topics WHERE forum_id IN (&lt;@forum ids&gt;)
  </code></pre>
  <p>I am wondering if there is a way to do <code>NOT IN</code> with that, like:</p>
  <pre><code>SELECT * FROM topics WHERE forum_id NOT IN (&lt;@forum ids&gt;)
  </code></pre>