maximum recursion depth exceeded

[Frank Maruf]

In signal i have been trying to doing this . i'm new at Django.

help me with this:

class Examiner(models.Model):

 #just think i have saved  (Primary School Certificate) in to my database.

examiner_title = models.CharField(max_length=150)
slug = models.SlugField(max_length=100, unique=True,blank=True)

Students Model:

Sd_Examiner=models.ForeignKey(Examiner,null=True,on_delete=models.SET_NULL,blank=True)

and at Signal:

exam = Examiner.objects.get(examiner_title__contains="Primary")
exam_id = str(exam.id)
instance.Sd_Examiner_id=exam_id
instance.save()

i get maximum recursion depth exceeded

how can i get over with this?