django model on_delete pass self to models.SET() -

for example, have book model, each book has author , tag.

def get_authors_first_tag(book):     try:         tag = book.author.tags.first()     except:         return none     else:         return tag.id   class author(models.model):     name = models.charfield(max_length=50)     tags = models.manytomanyfield('tag')  class book(models.model):   name = models.charfield(max_length=50)   author = models.foreignkey(author)   tag =  models.foreignkey('tag', null=true, on_delete=models.set(get_authors_first_tag)) 

i want, when deleting tag item, book's tag set author's first tag. how this? thanks. doc page - https://docs.djangoproject.com/en/1.9/ref/models/fields/#django.db.models.set

update. signals didn't help

from django.db.models.signals import post_delete django.dispatch import receiver  class book(models.model):   # temporaryly set null on delete,    # becouse neds cannot use default behavior (models.cascade)   tag =  models.foreignkey('tag', null=true, on_delete=models.set_null)  @receiver(post_delete, sender=tag) def delete_tag(instance, **kwargs):     book in instance.book_set.all():         try:             book.tag = book.author.tags.first()             book.save()         except:             pass 

update2 final working solution is: (dirty, works)

@receiver(post_delete, sender=tag) def delete_tag(instance, **kwargs):     books = book.objects.all()     book in books:         # after tag item deleting, book.tag set none         # if boook's tag null, it's author's first tag         if not book.tag:             book.tag = book.author.tags.first()             book.save() 

the set() doesn't accept arguments, should use signal receiver:

from django.db.models.signals.pre_delete django.dispatch import receiver  @receiver(pre_delete, sender=tag) def reset_tag(sender, **kwargs):     tag = kwargs['instance']     book in books.filter(tag=tag):         book.tag = book.author.tags.first()         book.save() 

put code models.py or make sure file put receiver imported @ startup, receiver connected.