Transformations: Fix duplicate field names when joinBy follows groupBy (#104206)

packages/grafana-data/src/transformations/transformers/groupBy.test.ts

@@ -418,6 +418,7 @@ describe('GroupBy transformer', () => {
   it('should retain "time" field type when used as aggregation (max, etc)', async () => {
     const testSeries = toDataFrame({
       refId: 'A',
+      name: 'issues',
       fields: [
         { name: 'user', type: FieldType.string, values: ['A', 'B', 'A', 'B'] },
         { name: 'time', type: FieldType.time, values: [7, 2, 1, 5] },
@@ -458,6 +459,11 @@ describe('GroupBy transformer', () => {
       ];
 
       expect(result[0].refId).toEqual('A');
+
+      // adding a frame name can modify field auto-name behavior if a joinBy transformer follows, which transfers
+      // the frame name to field.labels.name and calculateFieldDisplayName() may start treating it as a single-label field
+      expect(result[0].name).toBeUndefined();
+
       expect(result[0].fields).toEqual(expected);
     });
   });

packages/grafana-data/src/transformations/transformers/groupBy.ts

@@ -133,7 +133,7 @@ export const groupByTransformer: DataTransformerInfo<GroupByTransformerOptions>
           }
 
           processed.push({
-            ...frame,
+            refId: frame.refId,
             fields,
             length: valuesByGroupKey.size,
           });