title: One-To-One relationship with mongoose?
body: <p>I am aware of how to set up a one to many relationship between Mongoose entities, with the following type of code : </p>
  
  <pre><code>friends: [{type: ObjectId, ref: 'User'}]
  </code></pre>
  
  <p>But what if I want to only have a one-to-one relationship - Such as which client this user is related to?</p>