title: How to deal with 'undefined' fields in Mongodb?
body: <p>The values of some fields became 'undefined' after I did an operation on the collection.
  It's like:</p>
  
  <pre><code>{
    id: 1,
    name: "mia",
    country: undefined,
    birthday: undefined
  }
  </code></pre>
  
  <p>I want to update the country and birthday fields to "Not known" if the values of them are undefined, for all the document in this collection. How to do it?</p>