django: how do I query based on GenericForeignKey's fields?

I'm new in using GenericForeignKey, and I couldn't make it to work in a query statement. The tables are roughly like the following:

class Ticket(models.Model):
    issue_ct = models.ForeignKey(ContentType, related_name='issue_content_type')
    issue_id = models.PositiveIntegerField(null=True, blank=True)
    issue = generic.GenericForeignKey('issue_ct', 'issue_id')

class Issue(models.Model):
    scan = models.ForeignKey(Scan)

A scan creates one issue, an issue generates some tickets, and I made Issue as a foreign key to Ticket table. Now I have a Scan object, and I want to query for all the tickets that related to this scan. I tried this first:

tickets = Tickets.objects.filter(issue__scan=scan_obj)

which doesn't work. Then I tried this:

issue = Issue.objects.get(scan=scan_obj)
content_type = ContentType.objects.get_for_model(Issue)
tickets = Tickets.objects.filter(content_type=content_type, issue=issue)

Still doesn't work. I need to know how to do these kind of queries in django? Thanks.