Elastic Search [Query Time Join] [has_child] Type [<type>] does not have parent mapping

ES-ParentMapping

// index type creation 

curl -XPUT 'http://localhost:9200/example/' -d '
index :
    number_of_shards : 3
    number_of_replicas : 2
'

curl -XPUT 'http://localhost:9200/example/document3/_mapping' -d '{"document3" : {"properties":{"label" : {"type" : "string", "store" : "yes", "indexed" : "analysed"},"description" : {"type" : "string", "store" : "yes", "indexed" : "analyzed"}}}}'

curl -XPUT 'http://localhost:9200/example/entity6/_mapping' -d '{"entity6" : {"properties":{"label" : {"type" : "string", "store" : "yes", "index" : "analyzed"},"description" : {"type" : "string", "store" : "yes", "index" : "analyzed"}}}}'

curl -XPUT 'http://localhost:9200/example/entity6/_mapping' -d '{"entity6" : {"_parent" : {"type" : "document3"}}}'

// Check the mappings
curl http://localhost:9200/example/entity6/_mapping
curl http://localhost:9200/example/document3/_mapping

//Indexing -> Java API 1000 document3 and 100 entity6
//document3
String json = "{" +
                    "\"_id\":\"docId"+i+"\"," +
                    "\"title\":\"Title"+i+"\"," +
                    "\"content\":\"A process that compacts the index and merges segments in order to improve query performance.\" +\n" +
                    "                    \" New secondary segment(s) are created to contain documents inserted into the collection after it has been optimized. \" +\n" +
                    "                    \"A Lucene collection must be optimized periodically to maintain satisfactory query performance. \" +\n" +
                    "                    \"Optimization is run on the master server only. An optimized index will give you a performance gain at query time of at least 10%. \" +\n" +
                    "                    \"This gain may be more on an index that has become fragmented over a period of time with many updates and no optimizations.\" +\n" +
                    "                    \" Optimizations require a much longer time than does the distribution of an optimized collection to all slaves. Vegeta"+i+"\"," +
                    "}";

// entity6
 String json = "{" +
                        "\"_parent\":\""+parentId+"\"," +
                        "\"label\":\"label"+i+labelContent.toString()+"\"," +
                        "\"description\":\"A process that compacts the index and merges segments in order to improve query performance." +
                        "                    New secondary segment(s) are created to contain documents inserted into the collection after it has been optimized." +
                        "                    A Lucene collection must be optimized periodically to maintain satisfactory query performance." +
                        "                    Optimization is run on the master server only. An optimized index will give you a performance gain at query time of at least 10%." +
                        "                    This gain may be more on an index that has become fragmented over a period of time with many updates and no optimizations." +
                        "                     Optimizations require a much longer time than does the distribution of an optimized collection to all slaves. Vegeta"+i+"\"" +
                        "}";
//querying

{"query":{"has_child":{"type":"entity6","score_type":"max","query":{"term":{"label":"vegeta"}}}}}

Exception :

nested: QueryParsingException[[exercise] [has_child] Type [entity6] does not have parent mapping