title: How do I update a single subelement in an array by two references
body: <p>I have a school document structure like this:</p>
  
  <pre><code>{
      "_id":"tnoMB2PhsPPQKcafz",
      // some other property here
      "students":[{
          "student":{
              "ref_id":"eaH7JsponapSWCGf6",
              "name":"name",
              "family":"family",
              "grade":12
          },
          // many other student here
  ]}
  </code></pre>
  
  <p>I want to update grade property for determined student in determined school but I don't know how to do this.</p>
  
  <p>For example I have school _id (<strong>tnoMB2PhsPPQKcafz</strong>) and student ref_id (<strong>eaH7JsponapSWCGf6</strong>) and want to update determined student grade from 12 to 18.</p>