inner hits: Fix bug that resolves parent docs properly as inner hit when inner hit is defined on has_parent query.

src/main/java/org/elasticsearch/search/fetch/innerhits/InnerHitsContext.java

@@ -29,7 +29,9 @@ import org.apache.lucene.search.join.BitDocIdSetFilter;
 import org.apache.lucene.util.BitSet;
 import org.apache.lucene.util.Bits;
 import org.elasticsearch.ExceptionsHelper;
+import org.elasticsearch.common.lucene.Lucene;
 import org.elasticsearch.common.lucene.search.AndFilter;
+import org.elasticsearch.index.fieldvisitor.SingleFieldsVisitor;
 import org.elasticsearch.index.mapper.DocumentMapper;
 import org.elasticsearch.index.mapper.Uid;
 import org.elasticsearch.index.mapper.internal.ParentFieldMapper;
@@ -37,6 +39,7 @@ import org.elasticsearch.index.mapper.internal.UidFieldMapper;
 import org.elasticsearch.index.mapper.object.ObjectMapper;
 import org.elasticsearch.index.query.ParsedQuery;
 import org.elasticsearch.index.search.nested.NonNestedDocsFilter;
+import org.elasticsearch.search.SearchHitField;
 import org.elasticsearch.search.fetch.FetchSubPhase;
 import org.elasticsearch.search.internal.FilteredSearchContext;
 import org.elasticsearch.search.internal.SearchContext;
@@ -249,16 +252,27 @@ public final class InnerHitsContext {
                 topDocsCollector = TopScoreDocCollector.create(topN);
             }
 
-            String field;
+            final String term;
-            ParentFieldMapper hitParentFieldMapper = documentMapper.parentFieldMapper();
+            final String field;
-            if (hitParentFieldMapper.active()) {
+            if (documentMapper.parentFieldMapper().active()) {
-                // Hit has a active _parent field and it is a child doc, so we want a parent doc as inner hits.
+                // Active _parent field has been selected, so we want a children doc as inner hits.
                 field = ParentFieldMapper.NAME;
+                term = Uid.createUid(hitContext.hit().type(), hitContext.hit().id());
             } else {
-                // Hit has no active _parent field and it is a parent doc, so we want children docs as inner hits.
+                // No active _parent field has been selected, so we want parent docs as inner hits.
                 field = UidFieldMapper.NAME;
+                SearchHitField parentField = hitContext.hit().field(ParentFieldMapper.NAME);
+                if (parentField != null) {
+                    term = parentField.getValue();
+                } else {
+                    SingleFieldsVisitor fieldsVisitor = new SingleFieldsVisitor(ParentFieldMapper.NAME);
+                    hitContext.reader().document(hitContext.docId(), fieldsVisitor);
+                    if (fieldsVisitor.fields().isEmpty()) {
+                        return Lucene.EMPTY_TOP_DOCS;
+                    }
+                    term = (String) fieldsVisitor.fields().get(ParentFieldMapper.NAME).get(0);
+                }
             }
-            String term = Uid.createUid(hitContext.hit().type(), hitContext.hit().id());
             Filter filter = new TermFilter(new Term(field, term)); // Only include docs that have the current hit as parent
             Filter typeFilter = documentMapper.typeFilter(); // Only include docs that have this inner hits type.
             context.searcher().search(

src/test/java/org/elasticsearch/search/innerhits/InnerHitsTests.java

@@ -444,6 +444,45 @@ public class InnerHitsTests extends ElasticsearchIntegrationTest {
 
     }
 
+    @Test
+    public void testInnerHitsOnHasParent() throws Exception {
+        assertAcked(prepareCreate("stack")
+                        .addMapping("question", "body", "type=string")
+                        .addMapping("answer", "_parent", "type=question", "body", "type=string")
+        );
+        List<IndexRequestBuilder> requests = new ArrayList<>();
+        requests.add(client().prepareIndex("stack", "question", "1").setSource("body", "I'm using HTTPS + Basic authentication to protect a resource. How can I throttle authentication attempts to protect against brute force attacks?"));
+        requests.add(client().prepareIndex("stack", "answer", "1").setParent("1").setSource("body", "install fail2ban and enable rules for apache"));
+        requests.add(client().prepareIndex("stack", "question", "2").setSource("body", "I have firewall rules set up and also denyhosts installed.\\ndo I also need to install fail2ban?"));
+        requests.add(client().prepareIndex("stack", "answer", "2").setParent("2").setSource("body", "Denyhosts protects only ssh; Fail2Ban protects all daemons."));
+        indexRandom(true, requests);
+
+        SearchResponse response = client().prepareSearch("stack")
+                .setTypes("answer")
+                .addSort("_uid", SortOrder.ASC)
+                .setQuery(
+                        boolQuery()
+                                .must(matchQuery("body", "fail2ban"))
+                                .must(hasParentQuery("question", matchAllQuery()).innerHit(new QueryInnerHitBuilder()))
+                ).get();
+        assertNoFailures(response);
+        assertHitCount(response, 2);
+
+        SearchHit searchHit = response.getHits().getAt(0);
+        assertThat(searchHit.getId(), equalTo("1"));
+        assertThat(searchHit.getType(), equalTo("answer"));
+        assertThat(searchHit.getInnerHits().get("question").getTotalHits(), equalTo(1l));
+        assertThat(searchHit.getInnerHits().get("question").getAt(0).getType(), equalTo("question"));
+        assertThat(searchHit.getInnerHits().get("question").getAt(0).id(), equalTo("1"));
+
+        searchHit = response.getHits().getAt(1);
+        assertThat(searchHit.getId(), equalTo("2"));
+        assertThat(searchHit.getType(), equalTo("answer"));
+        assertThat(searchHit.getInnerHits().get("question").getTotalHits(), equalTo(1l));
+        assertThat(searchHit.getInnerHits().get("question").getAt(0).getType(), equalTo("question"));
+        assertThat(searchHit.getInnerHits().get("question").getAt(0).id(), equalTo("2"));
+    }
+
     @Test
     public void testParentChildMultipleLayers() throws Exception {
         assertAcked(prepareCreate("articles")