Tracking bugs reported for a Story

Tracking bugs reported for a Story

If I add bugs under a Story, I cannot pick up individual bug for fixing in current or future sprint; when I drag and drop a bug, the Story gets dropped along with all other subitems. . If I add a bug as independent item, there is no way to Tag associated story to it.
How do I tag Story to the bugs being added so that I can count which Story had how many bugs?