title: Mongoose ask for documents not referenced by another document
body: <p>I have the following collection structures using MongoDB and Mongoose:</p>
  
  <pre><code>const UserSchema = 
  {
    "name": {
      "type": "String",
      "required": true,
      "unique": true
    },
    "company_id": {
      "type": "ObjectId"
      "ref": "Company"
      "required": true
     }
  
  const CompanySchema = 
  {
    "name": {
      "type": "String",
      "required": true,
      "unique": true
    },
    "ein": {
      "type": "String"
    }
  }
  </code></pre>
  
  <p>What is the fastest way to get all companies that are not referenced by any user (all companies that have no users)?</p>
  
  <p>My first tought is:</p>
  
  <pre><code>User.find({}).exec()
  .then(users =&gt; {
      Company.find({ id: { $in: users}}).exec()
  })
  .then(companiesWithoutRefs =&gt; {
      return companiesWithoutRefs;
  })
  .catch(err =&gt; {
     throw new err;
  });
  </code></pre>
  
  <p>Questions:</p>
  
  <ol>
  <li><p>Are the promises structured correctly?</p></li>
  <li><p>Do I need to convert the $in: users statement to ObjectId? How to do so with multiple values?</p></li>
  </ol>
  
  <p>And the last and most important one:</p>
  
  <ol start="3">
  <li>Is there a way to do this query without fully loading the User collection, something smarter?</li>
  </ol>
  
  <p>thanks for helping.</p>