would like an example of Django ForeignKey.limit_choices_to with query

Hi -

a question from a django newbee ...

models.py looks something like this:

class Code_Type(models.Model):
    name= models.CharField(maxlength=20)
    description= models.CharField(maxlength=200)
    active_flag= models.BooleanField(default=True)

class Code_Value(models.Model):
    code_type = models.ForeignKey(Code_Type)
    name= models.CharField(maxlength=20)
    description= models.CharField(maxlength=200)
    active_flag= models.BooleanField(default=True)

When adding a Code_Value in the admin interface, I'd like the available choices for code_type be limited to those where active_flag = True ... this seems simple and straightforward, but I'm banging my head on the desk trying to get it to work.

An example would be a huge help - thanks!