title: Search documents which has specific column in mongo
body: <pre><code>db.posts.insert({
      title: "Hello World",
      "2d": [1,2],
      "3d": [3,4,5],
      "4d": [6,7,8,9],
      "6d: [1,2,3,4,5,6]
  });
  </code></pre>
  
  <p>Each document can have only the <code>2d</code> column, or just the <code>3d</code> column, or just the both, or just <code>4d</code> and <code>2d</code>, etc. How do I find documents which has that specific column?</p>
  
  <p><code>SELECT * FROM posts where posts.hascolumn=2d</code></p>
  
  <p>^Just for demonstration.</p>