How to delete a referenced object when the object, referencing the other object, is deleted?

I posted the following question on SO (link) and I haven't received any helpful answers. Can someone here please help me?

I am relatively new to Django and I'm just starting to get a feel for it, but I just can't work this out.

Many thanks!!!

I have the following model:

class Todo(models.Model):      user = models.OneToOneField(User)      note = models.CharField(max_length=255)      is_important = models.BooleanField(default=False)      is_complete = models.BooleanField(default=False)      reminder = models.OneToOneField(Reminder, blank=True, null=True, on_delete=models.SET_NULL)      class Reminder(models.Model):      start_time = models.DateTimeField()      stop_time = models.DateTimeField(blank=True)

Basically, a Todo becomes a Reminder when a start and optional end time are supplied.

At the moment, when I delete a Reminder object, the reminder field in the Todo object is set to Null, which what I want.

What I want to know:

How can I setup these models so that if a Todo object is deleted, the corresponding Reminder object will also be deleted?

Also, if it wasn't a one-to-one relationship, let's say it was a many-to-one (many Todo's to one Reminder) relationship, how could one setup the models so that if a Todo object was deleted, the Reminder object will also be deleted, but only if there were no more Todo objects linked to the Reminder?

Also, with regards to:

stop_time = models.DateTimeField(blank=True)

If it is left blank in the form, what will the default value be, stored in the database?

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