Make sure that field collapsing supports field aliases.

rest-api-spec/src/main/resources/rest-api-spec/test/search/110_field_collapsing.yml

@@ -1,7 +1,14 @@
 setup:
   - do:
       indices.create:
-          index:  test
+        index:  test
+        body:
+          mappings:
+            test:
+              properties:
+                numeric_group: { type: integer }
+                group_alias: { type: alias, path: numeric_group }
+
   - do:
       index:
           index:  test
@@ -341,3 +348,25 @@ setup:
   - match: { hits.hits.2.inner_hits.sub_hits.hits.hits.0._version: 55 }
   - match: { hits.hits.2.inner_hits.sub_hits.hits.hits.1._id: "4" }
   - match: { hits.hits.2.inner_hits.sub_hits.hits.hits.1._version: 44 }
+
+---
+"field collapsing on a field alias":
+  - skip:
+      version: " - 6.99.99"
+      reason: The associated bugfix is pending backport to 6.x.
+  - do:
+      search:
+        index: test
+        body:
+          collapse: { field: group_alias, inner_hits: { name: sub_hits } }
+          sort: [{ sort: desc }]
+
+  - match: { hits.total: 6 }
+  - length: { hits.hits: 3 }
+
+  - match: { hits.hits.0.fields.group_alias: [3] }
+  - match: { hits.hits.0.inner_hits.sub_hits.hits.total: 1}
+  - match: { hits.hits.1.fields.group_alias: [1] }
+  - match: { hits.hits.1.inner_hits.sub_hits.hits.total: 3}
+  - match: { hits.hits.2.fields.group_alias: [25] }
+  - match: { hits.hits.2.inner_hits.sub_hits.hits.total: 2}

server/src/main/java/org/elasticsearch/search/collapse/CollapseBuilder.java

@@ -247,6 +247,6 @@ public CollapseContext build(SearchContext context) {
                 + field + "`, " + "only indexed field can retrieve `inner_hits`");
         }
 
-        return new CollapseContext(fieldType, innerHits);
+        return new CollapseContext(field, fieldType, innerHits);
     }
 }

server/src/main/java/org/elasticsearch/search/collapse/CollapseContext.java

@@ -25,24 +25,29 @@
 import org.elasticsearch.index.mapper.NumberFieldMapper;
 import org.elasticsearch.index.query.InnerHitBuilder;
 
-import java.util.Collections;
 import java.util.List;
 
 /**
  * Context used for field collapsing
  */
 public class CollapseContext {
+    private final String fieldName;
     private final MappedFieldType fieldType;
     private final List<InnerHitBuilder> innerHits;
 
-    public CollapseContext(MappedFieldType fieldType, InnerHitBuilder innerHit) {
+    public CollapseContext(String fieldName,
+                           MappedFieldType fieldType,
+                           List<InnerHitBuilder> innerHits) {
+        this.fieldName = fieldName;
         this.fieldType = fieldType;
-        this.innerHits = Collections.singletonList(innerHit);
+        this.innerHits = innerHits;
     }
 
-    public CollapseContext(MappedFieldType fieldType, List<InnerHitBuilder> innerHits) {
-        this.fieldType = fieldType;
-        this.innerHits = innerHits;
+    /**
+     * The requested field name to collapse on.
+     */
+    public String getFieldName() {
+        return fieldName;
     }
 
     /** The field type used for collapsing **/

server/src/main/java/org/elasticsearch/search/fetch/subphase/DocValueFieldsFetchSubPhase.java

@@ -61,7 +61,7 @@ public void hitsExecute(SearchContext context, SearchHit[] hits) throws IOExcept
 
         if (context.collapse() != null) {
             // retrieve the `doc_value` associated with the collapse field
-            String name = context.collapse().getFieldType().name();
+            String name = context.collapse().getFieldName();
             if (context.docValueFieldsContext() == null) {
                 context.docValueFieldsContext(new DocValueFieldsContext(
                         Collections.singletonList(new FieldAndFormat(name, DocValueFieldsContext.USE_DEFAULT_FORMAT))));