title: Modify a field using the values of the other fields in the document
body: <h2>Initial Data</h2>
  <pre class="lang-js prettyprint-override"><code>db.members.insertMany([
     { &quot;_id&quot; : 1, &quot;member&quot; : &quot;abc123&quot;, &quot;status&quot; : &quot;A&quot;, &quot;points&quot; : 2, &quot;misc1&quot; : &quot;note to self: confirm status&quot;, &quot;misc2&quot; : &quot;Need to activate&quot;, &quot;lastUpdate&quot; : ISODate(&quot;2019-01-01T00:00:00Z&quot;) },
     { &quot;_id&quot; : 2, &quot;member&quot; : &quot;xyz123&quot;, &quot;status&quot; : &quot;A&quot;, &quot;points&quot; : 60, &quot;misc1&quot; : &quot;reminder: ping me at 100pts&quot;, &quot;misc2&quot; : &quot;Some random comment&quot;, &quot;lastUpdate&quot; : ISODate(&quot;2019-01-01T00:00:00Z&quot;) }
  ])
  </code></pre>
  <h2>Update query</h2>
  <pre class="lang-js prettyprint-override"><code>db.members.update(
     { },
     [
        { $set: { status: &quot;Modified&quot;, comments: [ &quot;$misc1&quot;, &quot;$misc2&quot; ], lastUpdate: &quot;$$NOW&quot; } },
        { $unset: [ &quot;misc1&quot;, &quot;misc2&quot; ] }
     ],
     { multi: true }
  )
  </code></pre>
  <p>The above queries worked fine, I would like to know why we are passing the array instead of the object in the second parameter of <code>.update()</code>?</p>
  <p>Why we can not do this with plain objects?</p>