title: Mongodb update number field with arithmetic operator
body: <p>I have a collection with a number field named "reference". I want to update every document in the collection and add 1000 to the reference field.</p>
  
  <p>so if reference = 1 it will become 1001</p>
  
  <p>pseudo below:</p>
  
  <pre><code>db.item.updateMany({}, {$set:{reference: reference+1000}}
  </code></pre>
  
  <p>Any idea how best to achieve this?</p>