title: Rails: mongodb error when using $lt
body: <p>this code return an error</p>
  
  <pre><code>db[:zips].find(city: {$lt: 'd'}).limit(2).to_a.each{|r| pp r}
  </code></pre>
  
  <blockquote>
    <p>syntax error, unexpected '}', expecting end-of-input</p>
  </blockquote>
  
  <p>However, this code works well</p>
  
  <pre><code>db[:zips].find(city: {:$lt=&gt; 'd'}).limit(2).to_a.each{|r| pp r}
  </code></pre>
  
  <p>Why can not use :$lt like the first one?</p>