fix(cabana): show to-many relation list columns joined with commas

modules/cabana/query.go

@@ -580,16 +580,48 @@ func relatedSelect(row reflect.Value, controller pact.AdminController, col ListC
 		}
 		field = field.Elem()
 	}
+	column := listRelationColumn(controller, col.Relation, col.Select)
+	if field.Kind() == reflect.Slice {
+		return relatedSelectMany(field, column)
+	}
 	if field.Kind() != reflect.Struct {
 		return nil, false
 	}
-	selected := fieldByColumn(field, listRelationColumn(controller, col.Relation, col.Select))
+	selected := fieldByColumn(field, column)
 	if !selected.IsValid() || !selected.CanInterface() {
 		return nil, false
 	}
 	return selected.Interface(), true
 }
 
+// relatedSelectMany joins the selected column of every related record with
+// ", ", as Winter's list does for a to-many relation column. No related
+// records leave the key unset, so the list shows its empty placeholder.
+func relatedSelectMany(records reflect.Value, column string) (any, bool) {
+	values := make([]string, 0, records.Len())
+	for i := 0; i < records.Len(); i++ {
+		record := records.Index(i)
+		for record.Kind() == reflect.Pointer && !record.IsNil() {
+			record = record.Elem()
+		}
+		if record.Kind() != reflect.Struct {
+			continue
+		}
+		selected := fieldByColumn(record, column)
+		for selected.IsValid() && selected.Kind() == reflect.Pointer && !selected.IsNil() {
+			selected = selected.Elem()
+		}
+		if !selected.IsValid() || !selected.CanInterface() || selected.Kind() == reflect.Pointer {
+			continue
+		}
+		values = append(values, fmt.Sprint(selected.Interface()))
+	}
+	if len(values) == 0 {
+		return nil, false
+	}
+	return strings.Join(values, ", "), true
+}
+
 func listRelationColumn(controller pact.AdminController, relation, column string) string {
 	mapper, ok := controller.(pact.ListRelationColumnMapper)
 	if !ok || mapper == nil {