title: Using NOT operator with Squeel?
body: <p>How do I write a SQL query using Squeel?</p>
  
  <pre><code>    SELECT * FROM documents where (NOT document.deleted OR document.deleted IS NULL)
  </code></pre>
  
  <p>I tried:</p>
  
  <pre><code>    Document.where { (-deleted) | (deleted == nil) }
  </code></pre>
  
  <p>but get this error:</p>
  
  <pre><code>    NoMethodError: undefined method `|' for deleted.-@:Squeel::Nodes::KeyPath
  </code></pre>