public void testResetRootDocId() throws Exception {
    Directory directory = newDirectory();
    IndexWriterConfig iwc = new IndexWriterConfig(null);
    iwc.setMergePolicy(NoMergePolicy.INSTANCE);
    RandomIndexWriter indexWriter = new RandomIndexWriter(random(), directory, iwc);

    List<Document> documents = new ArrayList<>();

    // 1 segment with, 1 root document, with 3 nested sub docs
    Document document = new Document();
    document.add(
        new Field(UidFieldMapper.NAME, "type#1", UidFieldMapper.Defaults.NESTED_FIELD_TYPE));
    document.add(
        new Field(TypeFieldMapper.NAME, "__nested_field", TypeFieldMapper.Defaults.FIELD_TYPE));
    documents.add(document);
    document = new Document();
    document.add(
        new Field(UidFieldMapper.NAME, "type#1", UidFieldMapper.Defaults.NESTED_FIELD_TYPE));
    document.add(
        new Field(TypeFieldMapper.NAME, "__nested_field", TypeFieldMapper.Defaults.FIELD_TYPE));
    documents.add(document);
    document = new Document();
    document.add(
        new Field(UidFieldMapper.NAME, "type#1", UidFieldMapper.Defaults.NESTED_FIELD_TYPE));
    document.add(
        new Field(TypeFieldMapper.NAME, "__nested_field", TypeFieldMapper.Defaults.FIELD_TYPE));
    documents.add(document);
    document = new Document();
    document.add(new Field(UidFieldMapper.NAME, "type#1", UidFieldMapper.Defaults.FIELD_TYPE));
    document.add(new Field(TypeFieldMapper.NAME, "test", TypeFieldMapper.Defaults.FIELD_TYPE));
    documents.add(document);
    indexWriter.addDocuments(documents);
    indexWriter.commit();

    documents.clear();
    // 1 segment with:
    // 1 document, with 1 nested subdoc
    document = new Document();
    document.add(
        new Field(UidFieldMapper.NAME, "type#2", UidFieldMapper.Defaults.NESTED_FIELD_TYPE));
    document.add(
        new Field(TypeFieldMapper.NAME, "__nested_field", TypeFieldMapper.Defaults.FIELD_TYPE));
    documents.add(document);
    document = new Document();
    document.add(new Field(UidFieldMapper.NAME, "type#2", UidFieldMapper.Defaults.FIELD_TYPE));
    document.add(new Field(TypeFieldMapper.NAME, "test", TypeFieldMapper.Defaults.FIELD_TYPE));
    documents.add(document);
    indexWriter.addDocuments(documents);
    documents.clear();
    // and 1 document, with 1 nested subdoc
    document = new Document();
    document.add(
        new Field(UidFieldMapper.NAME, "type#3", UidFieldMapper.Defaults.NESTED_FIELD_TYPE));
    document.add(
        new Field(TypeFieldMapper.NAME, "__nested_field", TypeFieldMapper.Defaults.FIELD_TYPE));
    documents.add(document);
    document = new Document();
    document.add(new Field(UidFieldMapper.NAME, "type#3", UidFieldMapper.Defaults.FIELD_TYPE));
    document.add(new Field(TypeFieldMapper.NAME, "test", TypeFieldMapper.Defaults.FIELD_TYPE));
    documents.add(document);
    indexWriter.addDocuments(documents);

    indexWriter.commit();
    indexWriter.close();

    IndexService indexService = createIndex("test");
    DirectoryReader directoryReader = DirectoryReader.open(directory);
    directoryReader =
        ElasticsearchDirectoryReader.wrap(directoryReader, new ShardId(indexService.index(), 0));
    IndexSearcher searcher = new IndexSearcher(directoryReader);

    indexService
        .mapperService()
        .merge(
            "test",
            new CompressedXContent(
                PutMappingRequest.buildFromSimplifiedDef("test", "nested_field", "type=nested")
                    .string()),
            MapperService.MergeReason.MAPPING_UPDATE,
            false);
    SearchContext searchContext = createSearchContext(indexService);
    AggregationContext context = new AggregationContext(searchContext);

    AggregatorFactories.Builder builder = AggregatorFactories.builder();
    NestedAggregatorBuilder factory = new NestedAggregatorBuilder("test", "nested_field");
    builder.addAggregator(factory);
    AggregatorFactories factories = builder.build(context, null);
    searchContext.aggregations(new SearchContextAggregations(factories));
    Aggregator[] aggs = factories.createTopLevelAggregators();
    BucketCollector collector = BucketCollector.wrap(Arrays.asList(aggs));
    collector.preCollection();
    // A regular search always exclude nested docs, so we use NonNestedDocsFilter.INSTANCE here
    // (otherwise MatchAllDocsQuery would be sufficient)
    // We exclude root doc with uid type#2, this will trigger the bug if we don't reset the root doc
    // when we process a new segment, because
    // root doc type#3 and root doc type#1 have the same segment docid
    BooleanQuery.Builder bq = new BooleanQuery.Builder();
    bq.add(Queries.newNonNestedFilter(), Occur.MUST);
    bq.add(new TermQuery(new Term(UidFieldMapper.NAME, "type#2")), Occur.MUST_NOT);
    searcher.search(new ConstantScoreQuery(bq.build()), collector);
    collector.postCollection();

    Nested nested = (Nested) aggs[0].buildAggregation(0);
    // The bug manifests if 6 docs are returned, because currentRootDoc isn't reset the previous
    // child docs from the first segment are emitted as hits.
    assertThat(nested.getDocCount(), equalTo(4L));

    directoryReader.close();
    directory.close();
  }