MongoDB sort by only exists entry, key with value first and key with null or not exists last
<p>I have a collection of 15000 documents. Some documents have <strong><code>sr_no</code></strong> with numeric values and other with absent of <strong><code>sr_no</code></strong>.</p>

<p>Now i want to get entries like all documents comes first which has <strong><code>sr_no</code></strong> with <strong>asc</strong> then all others.</p>

<p>I tried <strong><code>.find().sort({sr_no:1})</code></strong> but it return all <strong><code>null</code></strong> entries first then <strong>asc</strong> with <strong><code>sr_no</code></strong>.</p>

<blockquote>
  <p>This question seems too close with <a href="https://stackoverflow.com/questions/32680809/mongodb-sort-by-field-existing-and-then-alphabetically">duplicate</a>. But slightly defer
  with numeric key.</p>
</blockquote>

<p><strong>I answered it with hack below.</strong></p>