Fix: Correctly migrate mixed data source targets (#54152) (#54198)

* Failing test case

* Fix mixed datasource targets migration

(cherry picked from commit f9a49aa3ed)

Co-authored-by: Dominik Prokop <dominik.prokop@grafana.com>

public/app/features/dashboard/state/DashboardMigrator.test.ts

@@ -1814,6 +1814,12 @@ describe('DashboardModel', () => {
               {
                 datasource: 'prom',
               },
+              {
+                datasource: 'default',
+              },
+              {
+                datasource: null,
+              },
             ],
           },
           {
@@ -1848,6 +1854,8 @@ describe('DashboardModel', () => {
 
     it('should update target datasource props to refs', () => {
       expect(model.panels[2].targets[0].datasource).toEqual({ type: 'prometheus', uid: 'prom-uid' });
+      expect(model.panels[2].targets[1].datasource).toEqual({ type: 'prometheus', uid: 'prom2-uid' });
+      expect(model.panels[2].targets[2].datasource).toEqual({ type: 'prometheus', uid: 'prom2-uid' });
     });
 
     it('should update datasources in panels collapsed rows', () => {

public/app/features/dashboard/state/DashboardMigrator.ts

@@ -41,6 +41,7 @@ import { DatasourceSrv } from 'app/features/plugins/datasource_srv';
 import { isConstant, isMulti } from 'app/features/variables/guard';
 import { alignCurrentWithMulti } from 'app/features/variables/shared/multiOptions';
 import { CloudWatchMetricsQuery, LegacyAnnotationQuery } from 'app/plugins/datasource/cloudwatch/types';
+import { MIXED_DATASOURCE_NAME } from 'app/plugins/datasource/mixed/MixedDataSource';
 import { plugin as gaugePanelPlugin } from 'app/plugins/panel/gauge/module';
 import { plugin as statPanelPlugin } from 'app/plugins/panel/stat/module';
 
@@ -762,10 +763,14 @@ export class DashboardMigrator {
 
             for (const target of panel.targets) {
               if (target.datasource == null || target.datasource.uid == null) {
-                target.datasource = { ...panel.datasource };
+                if (panel.datasource?.uid !== MIXED_DATASOURCE_NAME) {
+                  target.datasource = { ...panel.datasource };
+                } else {
+                  target.datasource = migrateDatasourceNameToRef(target.datasource, { returnDefaultAsNull: false });
+                }
               }
 
-              if (panelDataSourceWasDefault && target.datasource.uid !== '__expr__') {
+              if (panelDataSourceWasDefault && target.datasource?.uid !== '__expr__') {
                 // We can have situations when default ds changed and the panel level data source is different from the queries
                 // In this case we use the query level data source as source for truth
                 panel.datasource = target.datasource as DataSourceRef;