title: how to update a number field using another number field in MongoDB
body: <p>Can MongoDB allow updating a number field using another number field in same document ?
  Let say I have a document like below :</p>
  
  <pre><code>{
     "_id" : "1",
     "a" : 2,
     "b" : 3
  }
  </code></pre>
  
  <p><strong>a</strong> and <strong>b</strong> fields are number fields(decimal,int...)
  my goal is update field <strong>b</strong> using field <strong>a</strong>
  How can I set <strong>b</strong> like <strong>b = 2*a</strong>.
  Is this possible ?</p>