title: what't the meaning of mongo $minKey?
body: <p>this page :
  <a href="https://docs.mongodb.com/manual/reference/operator/query/type/" rel="nofollow noreferrer">https://docs.mongodb.com/manual/reference/operator/query/type/</a></p>
  <pre><code>{ &quot;date&quot;: new Date(1393804800000), &quot;grade&quot;: MaxKey(), &quot;score&quot;: 2 },
  </code></pre>
  <p>when i show Maxkey() in  mongo shell:</p>
  <pre><code>MaxKey().help
  The MaxKey BSON Class.
  For more information on usage: https://mongodb.github.io/node-mongodb-native/3.6/api/MaxKey.html
  </code></pre>
  <p>how can I understand it ?
  should I compare it with &quot;$lt&quot;  or &quot;$gt&quot; like this ?</p>
  <pre><code>db.test.find({&quot;grades.grade&quot;: {&quot;$gt&quot;:&quot;a&quot;}})
  </code></pre>