MongoDB 查找指定字段值的字符串长度大于某个值的文档

## 解决过程

Google 关键词：`mongodb find documents where string length larger than`。

第二个搜索结果 [Find Strings greater than a particular length in MongoDB?](https://www.tutorialspoint.com/find-strings-greater-than-a-particular-length-in-mongodb) 解决了问题：

```js
db.stringFieldLengthDemo.find({ $where: 'this.UserName.length >11' })
```