Django many to many relation, include all IDs in queryset in both directions

Question

I have 2 models connected via M2M relation

class Paper(models.Model):
  title = models.CharField(max_length=70)
  authors = models.ManyToManyField(B, related_name='papers')

class Author():
  name = models.CharField(max_length=70)

• Is there a way to include authors as all related authors' IDs (and maybe name somehow)?

• Is there a way to include papers IDs as reverse relation (and maybe title as well)?

Author.objects.all().annotate(related_papers=F('papers'))

this only adds id of one paper, first one it finds I think.

Furthermore, changing related_papers to papers gives an error:

ValueError: The annotation ‘papers’ conflicts with a field on the
model.

Answer 1

From what I understand in your comments, you're using DRF. I will give you 2 answers.

1) If you're talking about model serializer, you can use PrimaryKeyRelatedField :

class AuthorSerializer(serializers.ModelSerializer):
    papers=serializers.PrimaryKeyRelatedField(many=True, read_only=True)

    class Meta:
        model = Author
        fields = ['name', 'papers']

class PaperSerializer(serializers.ModelSerializer):
    class Meta:
        model = Paper
        fields = '__all__'

This will return the IDs for the other side of the relationship whether you're on Paper or Author side. That will return the primary keys, not a representation of the object itself.

2) Now you're also talking about performance (e.g. database hit at each iteration).

Django (not DRF-specific) has a queryset method to handle preloading related objects. It's called prefetch_related.

For example, if you know you're going to need the relation object attributes and want to avoid re-querying the database, do as follow:

Author.objects.all().prefetch_related('papers') 
# papers will be already loaded, thus won't need another database hit if you iterate over them.

Answer 2

Actually, it has already been implemented for you. You should include a Many-to-Many relationship to author in your Paper model like this:

class Paper(models.Model):
  title = models.CharField(max_length=70)
  authors = models.ManyToManyField(Author, related_name='papers')

That gives you the opportunity to add Author objects to a related set using p.authors.add(u), assuming that p is the object of Paper model, and a is an object of Author model.

• You can access all related authors of a Paper instance using p.authors.all().
• You can access all related papers of an Author instance using u.papers.all().

This will return an instance of QuerySet that you can operate on.

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