Alerting: improve alerting default datasource search when extracting alerts (#29993)

* improve alerting search datasource

* correct the xorm get usage

* adding default datasource unittest

pkg/services/sqlstore/datasource.go

@@ -24,6 +24,7 @@ func init() {
 	bus.AddHandler("sql", UpdateDataSource)
 	bus.AddHandler("sql", GetDataSourceById)
 	bus.AddHandler("sql", GetDataSourceByName)
+	bus.AddHandler("sql", GetDefaultDataSource)
 }
 
 func getDataSourceByID(id, orgID int64, engine *xorm.Engine) (*models.DataSource, error) {
@@ -77,6 +78,20 @@ func GetDataSources(query *models.GetDataSourcesQuery) error {
 	return sess.Find(&query.Result)
 }
 
+// GetDefaultDataSource is used to get the default datasource of organization
+func GetDefaultDataSource(query *models.GetDefaultDataSourceQuery) error {
+	datasource := models.DataSource{}
+
+	exists, err := x.Where("org_id=? AND is_default=?", query.OrgId, true).Get(&datasource)
+
+	if !exists {
+		return models.ErrDataSourceNotFound
+	}
+
+	query.Result = &datasource
+	return err
+}
+
 func DeleteDataSourceById(cmd *models.DeleteDataSourceByIdCommand) error {
 	return inTransaction(func(sess *DBSession) error {
 		var rawSQL = "DELETE FROM data_source WHERE id=? and org_id=?"