title: MongoDB: How to get all documents by the keys from another collections without de-normalization?
body: <p>Imagine we have two Collections: one of <code>topics</code> and one of <code>posts</code>. And we have a <code>user._id</code>:</p>
  
  
  
  <pre class="lang-js prettyprint-override"><code>topic  {
      _id, // topicID
      userId
  }
  
  post {
      topicId
  }
  
  user {
      _id
  }
  </code></pre>
  
  <p>How in MongoDB get all the <code>posts</code> by specific <code>user</code> (<em>without storing <code>userId</code> in the <code>post</code></em>)?</p>
  
  <pre class="lang-js prettyprint-override"><code>(post.topicId === topic._id) &amp;&amp; (topic.userId === user._id)
  </code></pre>