Persists index field on agg type change if possible (#2227)

Signed-off-by: Ashwin Pc <ashwinpc@amazon.com>

src/plugins/wizard/public/application/components/data_tab/secondary_panel.tsx

@@ -4,15 +4,15 @@
  */
 
 import React, { useCallback, useMemo, useState } from 'react';
-import { cloneDeep } from 'lodash';
+import { cloneDeep, get } from 'lodash';
 import { useDebounce } from 'react-use';
 import { useTypedDispatch, useTypedSelector } from '../../utils/state_management';
 import { DefaultEditorAggParams } from '../../../../../vis_default_editor/public';
 import { Title } from './title';
 import { useIndexPatterns, useVisualizationType } from '../../utils/use';
 import { useOpenSearchDashboards } from '../../../../../opensearch_dashboards_react/public';
 import { WizardServices } from '../../../types';
-import { IAggType } from '../../../../../data/public';
+import { AggParam, IAggType, IFieldParamType } from '../../../../../data/public';
 import { saveDraftAgg, editDraftAgg } from '../../utils/state_management/visualization_slice';
 import { setValidity } from '../../utils/state_management/metadata_slice';
 
@@ -109,6 +109,20 @@ export function SecondaryPanel() {
           }}
           onAggTypeChange={function (aggId: string, aggType: IAggType): void {
             aggConfig.type = aggType;
+
+            // Persist field if the new agg type supports the existing field
+            const fieldParam = (aggType.params as AggParam[]).find(({ type }) => type === 'field');
+            if (fieldParam) {
+              const availableFields = (fieldParam as IFieldParamType).getAvailableFields(aggConfig);
+              const indexField = availableFields.find(
+                ({ name }) => name === get(draftAgg, 'params.field')
+              );
+
+              if (indexField) {
+                aggConfig.params.field = indexField;
+              }
+            }
+
             dispatch(editDraftAgg(aggConfig.serialize()));
           }}
         />