title: Update field with another field's value in the document
body: <p>I have a collection <code>t1</code> with the following fields in its schema</p>
  
  <pre><code>_id, field1, field1
  </code></pre>
  
  <p>I want set <code>field2</code>'s value <code>field1</code> like sql:</p>
  
  <pre><code>update t1 set field1=field2;
  </code></pre>
  
  <p>How do I do it in MongoDB? </p>