Elasticsearch 8.3创建join索引报unknown key [] for create index错误

该报错与join字段配置、Elasticsearch版本差异均无关系，属于Kibana Dev Tools的请求写法错误。
Kibana Dev Tools控制台不会将请求路径行末尾括号包裹的内容识别为注释，你写在路径后的(notice that ?refresh has been removed)、(notice that &refresh have been removed)这类备注，会被直接拼接进请求URL发送给Elasticsearch，导致URL解析逻辑异常，触发unknown key [] for create index错误。
你本地7.9.1版本测试时相同代码可正常运行，本质是测试时未携带这些行尾括号备注，和版本本身的兼容性无关。

删除所有请求路径行末尾的括号备注内容即可，routing、refresh都是join场景下写入文档的合法参数，不需要移除。修正后的可运行代码如下：

PUT my-index-000001
{
  "mappings": {
    "properties": {
      "my_id": {
        "type": "keyword"
      },
      "my_join_field": { 
        "type": "join",
        "relations": {
          "question": "answer" 
        }
      }
    }
  }
}

PUT my-index-000001/_doc/1?refresh
{
  "my_id": "1",
  "text": "This is a question",
  "my_join_field": {
    "name": "question" 
  }
}

PUT my-index-000001/_doc/2?refresh
{
  "my_id": "2",
  "text": "This is another question",
  "my_join_field": {
    "name": "question"
  }
}

PUT my-index-000001/_doc/3?routing=1&refresh
{
  "my_id": "3",
  "text": "This is an answer",
  "my_join_field": {
    "name": "answer", 
    "parent": "1" 
  }
}

PUT my-index-000001/_doc/4?routing=1&refresh
{
  "my_id": "4",
  "text": "This is another answer",
  "my_join_field": {
    "name": "answer",
    "parent": "1"
  }
}

• Kibana Dev Tools仅支持单独占行、以#开头的注释格式，不要将备注写在请求路径、请求体的代码行末尾，否则会被当成有效请求内容解析。
• Elasticsearch 8.x版本的官方云试用服务默认配置完全支持join字段类型，无需额外调整参数，子文档写入时强制要求携带routing参数指向父文档ID，属于正常的语法要求，不会触发报错。

内容的提问来源于stack exchange，提问作者Bukz