Sorting parents on property of inner hits

[Syed_ali_abbas_shah]

We have an index with Parent and Child Model, users and posts. Now we have a has_child query on posts, to be able to sort users based on desc order of post creation. in other words I need the user on top which has very latest post.

Here is my mapping

{
    "properties":{
          "id": {
            "type": "keyword"
          },
         "content":{
            "type": "text"
          },
          "created_at": {
            "type": "date"
        },
          "join_type": {
            "type": "join",
            "relations": {
                "user": [
                    "post",
                ],
            }
        }
    }
}

The following query does not sort my users, based on the inner_hits:

{
  "query": {
    "has_child": {
      "type": "post",
      "inner_hits" : {},
      "query": {
        "match" : {"content" : "test"}
      }
    }
  },
  "sort": [{"post.created_at" : "desc"}]
}