[Answer]-Django model without primary and unique field

Don’t create a model for this. Instead, add a many-to-many relation in either the Url and Monitor model.

Django will then maintain this intermediate table for you.

You probably want to make composite primary key, but django doesn’t support it:
https://code.djangoproject.com/wiki/MultipleColumnPrimaryKeys

What you could do is to let it create id field, but also add unique constraint like this:

class MonitorForUrl(models.Model):
    url = models.ForeignKey(Url, on_delete=models.CASCADE)
    monitor = models.ForeignKey(Monitor, on_delete=models.CASCADE)

    class Meta:
        db_table = 'monitors_for_url'
        constraints = [
            models.UniqueConstraint(
                fields=['url', 'monitor']
                name='unique_url_monitor'
            )
        ]

Neither field will be unique, but combination of the fields will.