title: MongoDB ObjectId foreign key implementation recommendation
body: <p>I'm looking for a recommendation on how best to implement MongoDB foreign key <code>ObjectId</code> fields. There seem to be two possible options, either containing the nested <code>_id</code> field or without. </p>
  
  <p>Take a look at the <strong>fkUid</strong> field below.</p>
  
  <pre><code>{'_id':ObjectId('4ee12488f047051590000000'), 'fkUid':{'_id':ObjectId('4ee12488f047051590000001')} } 
  </code></pre>
  
  <p>OR </p>
  
  <pre><code>{'_id':ObjectId('4ee12488f047051590000000'), 'fkUid':ObjectId('4ee12488f047051590000001')} }
  </code></pre>
  
  <p>Any recommendations would be much appreciated.</p>