title: jquery get form id by input id
body: <p>I have really basic question.
  How can I get form id by input element id.</p>
  
  <pre><code>&lt;form id="my_form"&gt;
      &lt;fieldset&gt;
          &lt;!--some &lt;div&gt;'s--&gt;
              &lt;input id="my_input"&gt;&lt;/div&gt;
          &lt;!--some &lt;div&gt;'s end--&gt;
      &lt;/fieldset&gt;
  &lt;/form&gt;
  </code></pre>
  
  <p>Now if I have</p>
  
  <pre><code>var form_input = $('#my_input');
  </code></pre>
  
  <p>How can I get id "my_form"?</p>
  
  <p>Thank you.</p>