Reindex parent/child relationship from 2.x to 6.x in elasticsearch - elasticsearch

Question

I need way to re-index parent/child data from 2.4 to 6.8 Join. The old index has multiple entities.
old-index mapping:
{
"mappings": {
"parentType": {
"properties": {
"field_1": {
"type": "text"
},
"field_2": {
"type": "text"
},
"field_3": {
"type": "keyword"
},
"filed_4": {
"type": "long"
}
}
},
"child_type_1": {
"_parent": {
"type": "case"
},
"_routing": {
" required": true
},
"child1_field_1": {
"type": "text"
},
"child1_field_2": {
"type": "text"
},
"child1_field_3": {
"type": "keyword"
}
},
"child_type_2": {
"_parent": {
"type": "parentType"
},
"_routing": {
"required": true
},
"child2_field_1": {
"type": "text"
},
"child2_field_2": {
"type": "text"
}
}
}
}
I want to transform it into to the following 6.8 mapping:
{
"mappings": {
"doc": {
"properties": {
"parentType": {
"properties": {
"field_1": {
"type": "text"
},
"field_2": {
"type": "text"
},
"field_3": {
"type": "keyword"
},
"filed_4": {
"type": "long"
}
}
},
"child_type_1": {
"child1_field_1": {
"type": "text"
},
"child1_field_2": {
"type": "text"
},
"child1_field_3": {
"type": "keyword"
}
},
"child_type_2": {
"child2_field_1": {
"type": "text"
},
"child2_field_2": {
"type": "text"
}
},
"join_field": {
"type": "join",
"relations": {
"parentType": [
"child_type_1",
"child_type_2"
]
}
}
}
}
}
}
I know I am supposed to use the re-index API but I am not sure how exactly the script has to be written. I want to re-index all the parent and children documents into the new index where the _type is "doc"