How to get document id (_id) inside an Elasticsearch native script - plugins

Question

I've got a native script that runs as a transform when adding a new document. I need the document id but its not passed in as part of the script params. Only the _source is passed in as the params, but not the _id value. Is there any way to get ElasticSearch to pass in the _id? Or some way of reading it ... somehow?
Below is a contrived ElasticSearch native script example that demonstrates what I'm talking about. setNextVar() is called by ElasticSearch and the "ctx" object is passed in. The value is a Map that only has one object in it, the _source object.
But the _id key/value pair is not passed in by ElasticSearch by default. I'm hoping there is a why to config the native script in the mapping json that tells it to pass in the document id.
public class ExampleNativeScript extends AbstractExecutableScript {
/**
* Factory class that serves native script to ElasticSearch
*/
public static class Factory extends AbstractComponent implements NativeScriptFactory {
#Inject
public Factory(Settings settings, String prefixSettings) {
super(settings, prefixSettings);
}
#Override
public ExecutableScript newScript(#Nullable Map<String, Object> params) {
return new ExampleNativeScript(params);
}
}
private final Map<String, Object> variables;
private Map ctx = null;
private Map source = null;
public ExampleNativeScript(Map<String, Object> params) {
this.variables = params;
}
#Override
public void setNextVar(String name, Object value) {
variables.put(name, value);
if (name.equals("ctx")) {
ctx = (Map<String, LinkedHashMap>) value;
source = (Map<String, LinkedHashMap>) ctx.get("_source");
} else {
//never gets here
System.out.println("Unexpected variable");
}
}
#Override
public Object run() {
//PROBLEM: ctx only has _source. _id does not get passed in so this blows chunks
String documentId = ctx.get("_id").toString();
System.out.println(documentId);
return ctx;
}
}

Answer 1

So just did some digging in the ElasticSearch source and found that its hard coded to just pass in the _source field, and none of the other ctx level fields. So there is no way to get/config ElasticSearch to pass in the _id
Below is the DocumentMapper.transformSourceAsMap() function that calls the native script's setNextVar() and run() functions. It shows that the only thing put in the ctx map is the _source field.
public Map<String, Object> transformSourceAsMap(Map<String, Object> sourceAsMap) {
try {
// We use the ctx variable and the _source name to be consistent with the update api.
ExecutableScript executable = scriptService.executable(language, script, scriptType, ScriptContext.Standard.MAPPING, parameters);
Map<String, Object> ctx = new HashMap<>(1);
ctx.put("_source", sourceAsMap);
executable.setNextVar("ctx", ctx);
executable.run();
ctx = (Map<String, Object>) executable.unwrap(ctx);
return (Map<String, Object>) ctx.get("_source");
} catch (Exception e) {
throw new ElasticsearchIllegalArgumentException("failed to execute script", e);
}
}