reindexing only child docs from a parent-join index to a new index in Elasticsearch 6.0

gistfile1.txt

# based on the v6 docs for parent-join
# https://www.elastic.co/guide/en/elasticsearch/reference/current/parent-join.html

DELETE /my_index
DELETE /my_index-new

PUT my_index
{
  "mappings": {
    "doc": {
      "properties": {
        "my_join_field": { 
          "type": "join",
          "relations": {
            "question": "answer" 
          }
        }
      }
    }
  }
}

PUT my_index-new
{
  "mappings": {
    "doc": {
      "properties": {
        "my_join_field": { 
          "type": "join",
          "relations": {
            "question": "answer" 
          }
        }
      }
    }
  }
}

PUT my_index/doc/1?refresh
{
  "text": "This is a question",
  "my_join_field": "question" 
}

PUT my_index/doc/2?refresh
{
  "text": "This is another question",
  "my_join_field": "question"
}

PUT my_index/doc/3?routing=1&refresh 
{
  "text": "This is an answer",
  "my_join_field": {
    "name": "answer", 
    "parent": "1" 
  }
}

PUT my_index/doc/4?routing=1&refresh
{
  "text": "This is another answer",
  "my_join_field": {
    "name": "answer",
    "parent": "1"
  }
}

GET /my_index/_search

# query to find all child docs
GET /my_index/_search
{
  "query": {
    "has_parent": {
      "parent_type": "question",
      "query": {
        "match_all": {}
      }
    }
  }
}

# use that query to reindex
POST _reindex
{
  "source": {
    "index": "my_index",
    "query": {
      "has_parent": {
        "parent_type": "question",
        "query": {
          "match_all": {}
        }
      }
    }
  },
  "dest": {
    "index": "my_index-new"
  }
}

# only 2 docs which are the child docs in the new index
GET my_index-new/_search