Reindexing - Null pointer exception while attempting to 'move' field into an object

[ttrmw (Thomas Millward Wright)]

Hey,

I'm attempting to change some mappings and reindex, whilst in the process changing my data's structure. My request looks like:

2017-07-18 16:56:48 +0100: POST http://127.0.0.1:9201/_reindex [status:500, request:0.009s, query:N/A]

2017-07-18 16:56:48 +0100: > {
    "source":{
        "index":"my_index"
    },
    "dest":{
        "index":"my_new_index"
    },
    "script":{
        "inline":"ctx._source.review.checked = ctx._source.remove(\"checked\"); ctx._source.review.false_positive = ctx._source.remove(\"false_positive\")"
    }
}

And I get:

[500] {"error":{
    "root_cause":[{
        "type":"script_exception",
        "reason":"runtime error",
        "script_stack":[{
            "ctx._source.review.checked = ctx._source.remove(\"checked\"); ",
            "                  ^---- HERE"],
            "script":"ctx._source.review.checked = ctx._source.remove(\"checked\"); ctx._source.review.false_positive = ctx._source.remove(\"false_positive\")",
            "lang":"painless"
        }],
        "type":"script_exception",
        "reason":"runtime error",
        "script_stack":[
            "ctx._source.review.checked = ctx._source.remove(\"checked\"); ",
            "                  ^---- HERE"
        ],
        "script":"ctx._source.review.checked = ctx._source.remove(\"checked\"); ctx._source.review.false_positive = ctx._source.remove(\"false_positive\")",
        "lang":"painless",
        "caused_by":{
            "type":"null_pointer_exception",
            "reason":null
        }
    },
    "status":500
}

My mapping in the target index is (forgive the ruby hash syntax):

{"my-index"=>
  {"mappings"=>
    {"mydoc"=>
      {"properties"=>
         "checked"=>{"type"=>"boolean"},
         "false_positive"=>{"type"=>"boolean"},
         "review"=>
          {"properties"=>
            {"checked"=>{"type"=>"boolean"},
             "false_positive"=>{"type"=>"boolean"},
             "reviewer"=>
              {"properties"=>
                {"email"=>
                  {"type"=>"text",
                   "fields"=>
                    {"keyword"=>{"type"=>"keyword", "ignore_above"=>256}}},
                 "id"=>
                  {"type"=>"text",
                  "fields"=>
                    {"keyword"=>{"type"=>"keyword", "ignore_above"=>256}}}}},
             "type"=>
              {"type"=>"text",
               "fields"=>
                {"keyword"=>{"type"=>"keyword", "ignore_above"=>256}}}}},\
                #...........

And in the source, lacks the 'review' object'. Any pointers on how to debug this? The error message is kind of opaque... Is it called _source that's breaking the painless script, or calling review (which wouldn't be present in the source index, I guess, since that's the point of the reindexing).

Thanks in advance!