title: node.js mongoose update specific value in document
body: <p>I have a document like below:</p>
  
  <pre><code>{
      "_id" : ObjectId("52e51809454115b7c5d593a4"),
      "usage" : {
          "from" : "2013-12-02T00:00:00+01:00",
          "to" : "2014-02-02T00:00:00+01:00",
          "used" : 674999999,
          "total" : NumberLong(4000000000)
      },
          "refills" : [ 
          {
              "id" : "RXh0cmEgU3VyZnZvbHlt",
              "amount" : 1000000000,
              "fee" : "5520",
              "feeVat" : "6900",
              "validTo" : "2014-02-02T00:00:00+01:00"
          }
      ],
  }
  </code></pre>
  
  <p>Where i would like to update the value "total" in "usage" and add to it the value "amount" in "refills"
  All other values should be the same.</p>
  
  <p>How do i do that best?</p>