title: Mongodb update type of field
body: <p>I have following mongodb document:</p>
  
  <pre><code>{ "_id" : ObjectId("51b73746485633a136000000"), "id_appointment" : 1602, "id_group" : "10", "action" : "SUPP", .....
  </code></pre>
  
  <p>I have to convert the field id_group into integer. How to do this on the command line?</p>
  
  <p>Thank you for any help.
  Andrea</p>