title: How to flatten a subdocument into root level in MongoDB?
body: <p>For example, if I have a document like this</p>
  <pre><code>{
    a: 1,
    subdoc: {
      b: 2,
      c: 3
    }
  }
  </code></pre>
  <p>How can I convert it into a format like this? <strong>(without using <code>project</code>)</strong></p>
  <pre><code>{
    a: 1,
    b: 2,
    c: 3
  }
  </code></pre>