Prometheus: Bug fix to check default expr is empty string to set legend format as auto (#69255)

* check default query.expr is empty string to set default legend format

* fix tests with default base query

* fix tests

public/app/plugins/datasource/prometheus/datasource.tsx

@@ -1265,12 +1265,7 @@ export class PrometheusDatasource
   }
 
   getDefaultQuery(app: CoreApp): PromQuery {
-    const defaults = {
-      refId: 'A',
-      expr: '',
-      range: true,
-      instant: false,
-    };
+    const defaults = promDefaultBaseQuery();
 
     if (app === CoreApp.UnifiedAlerting) {
       return {
@@ -1340,3 +1335,12 @@ export function prometheusRegularEscape(value: any) {
 export function prometheusSpecialRegexEscape(value: any) {
   return typeof value === 'string' ? value.replace(/\\/g, '\\\\\\\\').replace(/[$^*{}\[\]\'+?.()|]/g, '\\\\$&') : value;
 }
+
+export function promDefaultBaseQuery(): PromQuery {
+  return {
+    refId: 'A',
+    expr: '',
+    range: true,
+    instant: false,
+  };
+}

public/app/plugins/datasource/prometheus/querybuilder/components/PromQueryBuilderOptions.test.tsx

@@ -5,6 +5,7 @@ import { selectOptionInTest } from 'test/helpers/selectOptionInTest';
 
 import { CoreApp } from '@grafana/data';
 
+import { promDefaultBaseQuery } from '../../datasource';
 import { PromQuery } from '../../types';
 import { getQueryWithDefaults } from '../state';
 
@@ -106,7 +107,7 @@ function setup(queryOverrides: Partial<PromQuery> = {}, app: CoreApp = CoreApp.P
   const props = {
     app,
     query: {
-      ...getQueryWithDefaults({ refId: 'A' } as PromQuery, CoreApp.PanelEditor),
+      ...getQueryWithDefaults(promDefaultBaseQuery(), CoreApp.PanelEditor),
       ...queryOverrides,
     },
     onRunQuery: jest.fn(),

public/app/plugins/datasource/prometheus/querybuilder/state.test.ts

@@ -7,7 +7,7 @@ import { changeEditorMode, getQueryWithDefaults } from './state';
 
 describe('getQueryWithDefaults(', () => {
   it('should set defaults', () => {
-    expect(getQueryWithDefaults({ refId: 'A' } as PromQuery, CoreApp.Dashboard)).toEqual({
+    expect(getQueryWithDefaults({ expr: '', refId: 'A' } as PromQuery, CoreApp.Dashboard)).toEqual({
       editorMode: 'builder',
       expr: '',
       legendFormat: '__auto',
@@ -17,7 +17,7 @@ describe('getQueryWithDefaults(', () => {
   });
 
   it('should set both range and instant to true when in Explore', () => {
-    expect(getQueryWithDefaults({ refId: 'A' } as PromQuery, CoreApp.Explore)).toEqual({
+    expect(getQueryWithDefaults({ expr: '', refId: 'A' } as PromQuery, CoreApp.Explore)).toEqual({
       editorMode: 'builder',
       expr: '',
       legendFormat: '__auto',
@@ -29,7 +29,7 @@ describe('getQueryWithDefaults(', () => {
 
   it('should not set both instant and range for Prometheus queries in Alert Creation', () => {
     expect(
-      getQueryWithDefaults({ refId: 'A', range: true, instant: true } as PromQuery, CoreApp.UnifiedAlerting)
+      getQueryWithDefaults({ expr: '', refId: 'A', range: true, instant: true } as PromQuery, CoreApp.UnifiedAlerting)
     ).toEqual({
       editorMode: 'builder',
       expr: '',
@@ -45,13 +45,15 @@ describe('getQueryWithDefaults(', () => {
       expect(query.editorMode).toBe(QueryEditorMode.Code);
     });
 
-    expect(getQueryWithDefaults({ refId: 'A' } as PromQuery, CoreApp.Dashboard).editorMode).toEqual(
+    expect(getQueryWithDefaults({ expr: '', refId: 'A' } as PromQuery, CoreApp.Dashboard).editorMode).toEqual(
       QueryEditorMode.Code
     );
   });
 
   it('should return default editor mode when it is provided', () => {
-    expect(getQueryWithDefaults({ refId: 'A' } as PromQuery, CoreApp.Dashboard, QueryEditorMode.Code)).toEqual({
+    expect(
+      getQueryWithDefaults({ expr: '', refId: 'A' } as PromQuery, CoreApp.Dashboard, QueryEditorMode.Code)
+    ).toEqual({
       editorMode: 'code',
       expr: '',
       legendFormat: '__auto',

public/app/plugins/datasource/prometheus/querybuilder/state.ts

@@ -46,7 +46,8 @@ export function getQueryWithDefaults(
     result = { ...query, editorMode: getDefaultEditorMode(query.expr, defaultEditor) };
   }
 
-  if (query.expr == null) {
+  // default query expr is now empty string, set in getDefaultQuery
+  if (query.expr === '') {
     result = { ...result, expr: '', legendFormat: LegendFormatMode.Auto };
   }