title: How can I create unique IDs for embedded documents in MongoDB?
body: <p>So I need to reference particular subdocuments uniquely from items in my collection.  For instance:</p>
  
  <pre><code>User = {
      'name': 'jim',
      'documents: [
          {'id': 0001, 'title': "My document"},
          {'id': 0002, 'title': "My second document!"},
      ]
  }
  </code></pre>
  
  <p>So I need to be able to auto-create IDs for new documents, preferably not at the application level (since there will be race conditions in the actual development scenario).</p>
  
  <p>Is there a way to use mongo's autogenerated ObjectId (used in the _id field at the collection level), or something similar?</p>