Advert Snippet in wagtaildemo

models.py

# Advert Snippet

class AdvertPlacement(models.Model):
    page = ParentalKey('wagtailcore.Page', related_name='advert_placements')
    advert = models.ForeignKey('demo.Advert', related_name='+')


class Advert(models.Model):
    page = models.ForeignKey(
        'wagtailcore.Page',
        related_name='adverts',
        null=True,
        blank=True
    )
    url = models.URLField(null=True, blank=True)
    text = models.CharField(max_length=255)

    panels = [
        PageChooserPanel('page'),
        FieldPanel('url'),
        FieldPanel('text'),
    ]

    def __unicode__(self):
        return self.text

register_snippet(Advert)