title: Searching keywords inside the nested arrays
body: <p>I have an object and its array that contain multiple arrays like below</p>
  <pre><code>Obj A1:  {
         a: [
          [gta, lol, dota],
          [11, 33, 44],
          [a, b, c, d]
         ]
      }
  
  Obj A2:  {
         a: [
          [Starcraft],
          [11, 33, 44],
          [a, b, c, d]
         ]
      }
  </code></pre>
  <p>I need to return documents that have <code>dota</code> inside the array <code>a</code>. How can I do that with <code>.find()</code>? I used <code>.find({ a: /dota/i})</code>. However, it returns <code>undefined</code>.</p>