models: do not block deletion of request objects

polynum_blackboard/models.py

@@ -6,7 +6,7 @@ from polynum.base.models import Request, DocumentLicence
 import constant
 
 class BlackBoardPush(models.Model):
-    request = models.ForeignKey(Request)
+    request = models.ForeignKey(Request, on_delete=models.SET_NULL)
     course_name = models.TextField(max_length=64)
     visible_to_students = models.BooleanField(blank=True)
     sent = models.BooleanField(blank=True, default=False)