如何从 Spark DataFrame 生成 n 级分层 JSON

Question

给定以下 Spark 数据框：

+----+-----------+------+-------+
| id | parent_id | data | level |
+----+-----------+------+-------+
|  1 | null      | x    |     1 |
| 21 | 1         | y    |     2 |
| 22 | 1         | w    |     2 |
| 31 | 21        | z    |     3 |
+----+-----------+------+-------+

其中“level”表示树状结构中的级别。

如何生成 n 级分层 JSON？

{
  "id": "1",
  "data": "x",
  "items": [
    {
      "id": "21",
      "data": "y",
      "items": [
        {
          "id": "31",
          "data": "z",
          "items": []
        }
      ]
    },
    {
      "id": "22",
      "data": "w",
      "items": []
    }
  ]
}