title: Mongo find documents that do not contain a given value (using $not)
body: <p>I have two items in MongoDB:</p>
  
  <pre><code>{'title':'active item',
   'tags':[
          {'tag':'active'},
          {'tag':'anothertag'}
  ]}
  {'title':'completed item',
   'tags':[
          {'tag':'completed'}
  ]}
  </code></pre>
  
  <p>It works to find items which are tagged as completed:</p>
  
  <pre><code>db.items.find({'tags.tag':'completed'})
  RESULT: [&lt;completed item&gt;]
  </code></pre>
  
  <p>Now, I want to select all items which are not tagged as completed, so I tried:</p>
  
  <pre><code>db.items.find({$not:{'tags.tag':'completed'}})
  DESIRED RESULT: [&lt;active item&gt;]
  ACTUAL RESULT: []
  </code></pre>
  
  <p>But somehow this doesn't return any results.  Clearly I misunderstand $not in Mongo, but why?  How do I query to find the records that do not contain a given value in their tags?</p>