title: Mongoose update with limit
body: <p>I am looking to update <code>X documents</code> all at once.  The short is I basically need to randomly select <code>N documents</code> and then update them as <code>"selected"</code>.  I'm trying to design an API that needs to randomly distribute questions.  I can not find a way to do this in mongoose I have tried:</p>
  
  <p><code>update</code> ends up selecting everything</p>
  
  <pre><code>Question
    .update({}, {
      $inc: {
        answerCount: 1,
        lockedCount: 1
      },
      $push:{
        devices: deviceID
      }
  }, {multi:true})
   .limit(4)
  </code></pre>
  
  <p>--- I also tried</p>
  
  <pre><code>Question
  .find()
  .sort({
     answerCount: 1,
     lockedCount: 1
  })
  .limit(req.query.limit || 4)
  .update({}, {
      $inc: {
        answerCount: 1,
        lockedCount: 1
      },
      $push:{
        devices: deviceID
      }
  }, { multi: true },  callback);
  </code></pre>
  
  <p>Both resulted in updating all docs.  Is there a way to push this down to mongoose without having to use <code>map</code> ?  The other thing I did not mention is <code>.update()</code> without multi resulted in 1 document being updated.  </p>