Model Inheritance of ManyToMany Through - django

Question

Suppose I have following models:
class Author(Model):
name = CharField()
class Publication(Model):
name = CharField()
authors = ManyToManyField(Author)
class Meta:
abstract = True
class Book(Publication):
pass
class Article(Publication):
pass
class Journal(Publication):
pass
How to change code so that I can add through table to authors? If I write authors = ManyToManyField(Author, through='Relationship'), it will not work.

Answer 1

Django thankfully takes care of intermediate tables without any coding. You can even access them using .through on the M2M relationship manager, e.g. one_publication.authors.through.
You only need to specify the through table if you want to manage it yourself, e.g. because you want to add more fields than just the foreign keys of the two related entities. Is that the case here?
If yes, you have to create a Relationship model (consider giving it a more helpful name) that contains foreign keys to Publication and Author.
Update: If you want to add a default order to object lists from many-to-many relationships, an intermediate model would indeed be one way to achieve this:
class Relationship(models.Model):
author = models.ForeignKey(Author)
publication = models.ForeignKey(Publication)
# Any further fields that you need
class Meta:
ordering = ['author__last_name', 'author__first_name']
However, you can just as easily order your m2m relationships on querying them, without any intermediate model or through manager:
book = Book.objects.first()
for author in book.authors.order_by('last_name', 'first_name'):
# Will print in alphabetical order
print(f'Author: {author.first_name} {author.last_name}')
One caveat is that if you use prefetching, you need to specify the ordering in a Prefetch object to avoid executing the query twice, first for prefetching without ordering, than on access with ordering.
# 2 queries plus one for every book
books = Book.objects.prefetch_related('authors')
for book in books:
for author in book.authors.order_by('last_name', 'first_name'):
print(f'Author: {author.first_name} {author.last_name}')
# 2 queries regardless of number of books
books = Book.objects.prefetch_related(Prefetch('authors',
queryset=Author.objects.order_by('last_name', 'first_name')))
for book in books:
for author in book.authors.all():
print(f'Author: {author.first_name} {author.last_name}')