How to bulk update the many-to-many field of a Django queryset

Question

How does one update - in bulk - a many to many field in the queryset of a Django data model?

For instance, I have a data model called Photo, and another called PhotoStream. In Photo, I have which_stream = models.ManyToManyField(PhotoStream).

I extracted a queryset of Photos called childhood_photos, and I need to add a new PhotoStream object in the many-to-many fields of all objects within this queryset. Let's call this PhotoStream object for_classmates.

I try childhood_photos.update(which_stream.add(for_classmates)), but it gives me an error: global name 'which_stream' is not defined.

How can I do this operation?

Answer 1

You can access the through model of your m2n relationship via the field's .through attribute (see the documentation). That will allow you to bulk create the necessary through model instances:

through_model = Photo.which_stream.through  # gives you access to auto-created through model
# print through_model
# <class 'app_label.models.Photo_which_stream'>  # or sth. similar

through_model.objects.bulk_create([
    through_model(photo_id=pk, photostream_id=for_classmates.pk) 
        for pk in childhood_photos.values_list('pk', flat=True)
])