title: Firebase firestore complicated query
body: <p>I'm wondering if this is possible, and if it's a good solution to my problem.</p>
  
  <p>I want users to be able to <code>subscribe</code> to content. The content is associated with an <code>id</code>.. for instance:</p>
  
  <pre><code>'JavaScript': 1,
  'C++': 2,
  'Python': 3,
  'Java': 4,
  </code></pre>
  
  <p>Let's say a user subscribes to <code>1</code>, <code>3</code>, and <code>4</code>. </p>
  
  <p>So their user json data would appear as:</p>
  
  <p><code>'subscribed_to': [1,3,4]</code></p>
  
  <p>Now in my firestore, I have posts. Each post gets assigned a <code>content_id</code> (1-4 for instance), and so when I query for the content that this user is subscribed to, how would I do that so as effectively as possible? </p>