title: how can I create an array based some fields in mongodb?
body: <p>eg: the input is</p>
  
  <pre><code>{
      "_id" : ObjectId("5e79ae1c11344b2895797042"),
      "startDay" : "20200312",
      "endDay" : "20200314"
  }
  </code></pre>
  
  <p>I want to get ['20200312','20200313','20200314']. What should I do ?</p>