Improved explain support for nested query.

Closes #2503

src/main/java/org/elasticsearch/index/search/nested/BlockJoinQuery.java

@@ -408,9 +408,17 @@ public class BlockJoinQuery extends Query {
         public Explanation explain(int docBase) throws IOException {
             int start = docBase + prevParentDoc + 1; // +1 b/c prevParentDoc is previous parent doc
             int end = docBase + parentDoc - 1; // -1 b/c parentDoc is parent doc
-            return new ComplexExplanation(
+            ComplexExplanation explanation = new ComplexExplanation(
-                    true, score(), String.format(Locale.ROOT, "Score based on child doc range from %d to %d", start, end)
+                    true, score(), String.format(Locale.ROOT, "Score based on score mode %s and child doc range from %d to %d", scoreMode, start, end)
             );
+
+            for (int i = 0; i < childDocUpto; i++) {
+                int childDoc = pendingChildDocs[i];
+                float childScore = pendingChildScores[i];
+                explanation.addDetail(new Explanation(childScore, String.format(Locale.ROOT, "Child[%d]", childDoc)));
+            }
+
+            return explanation;
         }
 
     }

src/test/java/org/elasticsearch/test/integration/nested/SimpleNestedTests.java

@@ -19,6 +19,7 @@
 
 package org.elasticsearch.test.integration.nested;
 
+import org.apache.lucene.search.Explanation;
 import org.elasticsearch.action.admin.indices.status.IndicesStatusResponse;
 import org.elasticsearch.action.delete.DeleteResponse;
 import org.elasticsearch.action.get.GetResponse;
@@ -26,7 +27,6 @@ import org.elasticsearch.action.search.SearchResponse;
 import org.elasticsearch.action.search.SearchType;
 import org.elasticsearch.client.Client;
 import org.elasticsearch.index.query.FilterBuilders;
-import org.elasticsearch.index.query.FilteredQueryBuilder;
 import org.elasticsearch.index.query.QueryBuilders;
 import org.elasticsearch.search.facet.FacetBuilders;
 import org.elasticsearch.search.facet.termsstats.TermsStatsFacet;
@@ -517,4 +517,48 @@ public class SimpleNestedTests extends AbstractNodesTests {
         assertThat(client.prepareGet("test", "type1", "1").execute().actionGet().exists(), equalTo(false));
     }
 
+    @Test
+    public void testExplain() throws Exception {
+        client.admin().indices().prepareDelete().execute().actionGet();
+
+        client.admin().indices().prepareCreate("test")
+                .addMapping("type1", jsonBuilder().startObject().startObject("type1").startObject("properties")
+                        .startObject("nested1")
+                        .field("type", "nested")
+                        .endObject()
+                        .endObject().endObject().endObject())
+                .execute().actionGet();
+
+        client.admin().cluster().prepareHealth().setWaitForGreenStatus().execute().actionGet();
+
+        client.prepareIndex("test", "type1", "1").setSource(jsonBuilder().startObject()
+                .field("field1", "value1")
+                .startArray("nested1")
+                .startObject()
+                .field("n_field1", "n_value1")
+                .endObject()
+                .startObject()
+                .field("n_field1", "n_value1")
+                .endObject()
+                .endArray()
+                .endObject())
+                .setRefresh(true)
+                .execute().actionGet();
+
+        SearchResponse searchResponse = client.prepareSearch("test")
+                .setQuery(nestedQuery("nested1", termQuery("nested1.n_field1", "n_value1")).scoreMode("total"))
+                .setExplain(true)
+                .execute().actionGet();
+        assertThat(Arrays.toString(searchResponse.shardFailures()), searchResponse.failedShards(), equalTo(0));
+        assertThat(searchResponse.hits().totalHits(), equalTo(1l));
+        Explanation explanation = searchResponse.hits().hits()[0].explanation();
+        assertThat(explanation.getValue(), equalTo(2f));
+        assertThat(explanation.getDescription(), equalTo("Score based on score mode Total and child doc range from 0 to 1"));
+        assertThat(explanation.getDetails().length, equalTo(2));
+        assertThat(explanation.getDetails()[0].getValue(), equalTo(1f));
+        assertThat(explanation.getDetails()[0].getDescription(), equalTo("Child[0]"));
+        assertThat(explanation.getDetails()[1].getValue(), equalTo(1f));
+        assertThat(explanation.getDetails()[1].getDescription(), equalTo("Child[1]"));
+    }
+
 }