title: Mongoose document save fails with "TypeError: Cannot read property 'options' of undefined"
body: <p>I have a mongoose schema with a few "ref" properties. Recently (I believe after upgrading to a recent version of mongoose), my saves have started failing unless I populate() the ref fields. The error it fails with is</p>
  
  <pre><code>TypeError: Cannot read property 'options' of undefined
  </code></pre>
  
  <p>Has anyone else seen this? I suppose I can work around it by always populating, but I'd like to understand why this is necessary.</p>