title: Mongoose error on promise with save?
body: <p>When i try to get promise back with save operation on instance of model. i get error: <code>undefined is not a function</code></p>
  
  <pre><code>instance.save().exec().then(..)
  </code></pre>
  
  <p>However, if i try to get promise with model like this, then it works.</p>
  
  <pre><code>model.find(..).exec().then(..)
  </code></pre>
  
  <p>Is there no way to get promise for save action. Currently i just pass callback to save function. However, for the sake of consistency i'd like to do all db operations in the same manner. </p>