[7.12] [Discover] Do not set fieldsFromSource when not using fields API (#98575) (#98755)

* [Discover] Do not set fieldsFromSource when not using fields API (#98575)

* [Discover] Do not set fieldsFromSource when not using fields API

* Add Search Source unit test

* Add Search Source unit test

* Fix await error

Co-authored-by: Kibana Machine <42973632+kibanamachine@users.noreply.github.com>

src/plugins/data/common/search/search_source/search_source.test.ts

@@ -486,6 +486,13 @@ describe('SearchSource', () => {
         const request = await searchSource.getSearchRequestBody();
         expect(request.stored_fields).toEqual(['*']);
       });
+
+      test('_source is not set when using the fields API', async () => {
+        searchSource.setField('fields', ['*']);
+        const request = await searchSource.getSearchRequestBody();
+        expect(request.fields).toEqual(['*']);
+        expect(request._source).toEqual(false);
+      });
     });
 
     describe('source filters handling', () => {

src/plugins/discover/public/application/helpers/update_search_source.test.ts

@@ -136,4 +136,34 @@ describe('updateSearchSource', () => {
     ]);
     expect(volatileSearchSourceMock.getField('fieldsFromSource')).toBe(undefined);
   });
+
+  test('does not explicitly request fieldsFromSource when not using fields API', async () => {
+    const persistentSearchSourceMock = createSearchSourceMock({});
+    const volatileSearchSourceMock = createSearchSourceMock({});
+    const sampleSize = 250;
+    updateSearchSource({
+      persistentSearchSource: persistentSearchSourceMock,
+      volatileSearchSource: volatileSearchSourceMock,
+      indexPattern: indexPatternMock,
+      services: ({
+        data: dataPluginMock.createStartContract(),
+        uiSettings: ({
+          get: (key: string) => {
+            if (key === SAMPLE_SIZE_SETTING) {
+              return sampleSize;
+            }
+            return false;
+          },
+        } as unknown) as IUiSettingsClient,
+      } as unknown) as DiscoverServices,
+      sort: [] as SortOrder[],
+      columns: [],
+      useNewFieldsApi: false,
+      showUnmappedFields: false,
+    });
+    expect(persistentSearchSourceMock.getField('index')).toEqual(indexPatternMock);
+    expect(volatileSearchSourceMock.getField('size')).toEqual(sampleSize);
+    expect(volatileSearchSourceMock.getField('fields')).toEqual(undefined);
+    expect(volatileSearchSourceMock.getField('fieldsFromSource')).toBe(undefined);
+  });
 });

src/plugins/discover/public/application/helpers/update_search_source.ts

@@ -65,8 +65,6 @@ export function updateSearchSource({
       volatileSearchSource.setField('fields', [fields]);
     } else {
       volatileSearchSource.removeField('fields');
-      const fieldNames = indexPattern.fields.map((field) => field.name);
-      volatileSearchSource.setField('fieldsFromSource', fieldNames);
     }
   }
 }