models.py

def site_authors(request):
    return Author.objects.filter(site_id=request.site.id)

class Authorships(Orderable, models.Model):
    author = models.ForeignKey('Author', related_name='author_paper_relationship', on_delete=models.CASCADE)
    paper = ParentalKey('Page', related_name='paper_author_relationship', on_delete=models.CASCADE)

    panels = [
        SnippetChooserPanel('author', qs_restriction=site_authors, editable=False)
    ]

    class Meta:
        unique_together = ["author", "paper"]


#### and then I need to create code that adds a permissible kwarg to SnippetChooserPanel and figure out 
###  how to have it affect the query done in wagtailsnippets/views/chooser.py#choose