what does on_delete=models.PROTECT and on_delete=models.CASCADE do on Django models?

• CASCADE
  Cascade deletes. Django emulates the behavior of the
  SQL constraint ON DELETE CASCADE and also deletes the object
  containing the ForeignKey.

• PROTECT
  Prevent deletion of the referenced object by raising
ProtectedError, a subclass of django.db.IntegrityError.

the things get deleted because once you change your model you need to do makemigrations and migrate to see the change.