title: Mongo conditional for "key doesn't exist"?
body: <p>I want to find a document using a conditional if the key == None or if the key doesn't exist. Something like this:</p>
  
  <pre><code>myDoc = self.request.root.db.myDocs.find_one({
                            '$or': [
                                {'myKey' : $doesNotExist } ,
                                {'myKey' : None }
                              ]
                      })
  </code></pre>
  
  <p>I'd also like to be able to find a document just by a missing key like this:</p>
  
  <pre><code>myDoc = self.request.root.db.myDocs.find_one( {'myKey' : $doesNotExist } )
  </code></pre>
  
  <p>How can I accomplish this?</p>