Django-从另一个模型字段获取默认值

Question

I came across this strange problem in django where I have 3 models Books, Language, Book_language where i map book to its language.

from django.db import models

class Book(models.Model):
    title = models.CharField(max_length=200)
    year = models.IntegerField()

class Language(models.Model):
    name = models.CharField(max_length=50)

class Book_language(models.Model):
    book = models.ForeignKey(Book)
    language = models.ForeignKey(Language)
    other_title = models.CharField(max_length=200, default=Book._meta.get_field('title').get_default()) # not working

So far i am creating a book and with title and later assigning with language so the title is same for all languages, later i understand that the title may not be the same in all languages, so i want other_title to be default to title if not mention ( but not working ), and appear in django admin when i map with language.

Answer 1

can you simple override save method?

class Book_language(models.Model):
    book = models.ForeignKey(Book)
    language = models.ForeignKey(Language)
    other_title = models.CharField(max_length=200)

    def save(self, *args, **kwargs):
        if not self.other_title:
              self.other_title = self.book.title
        super(Book_language, self).save(*args, **kwargs)

for updating-multiple-objects-at-once previous empty data you can use expressions F :

from django.db.models import Q, F

empty_f = Q(other_title__isnull=True) | Q(other_title__exact='')
for bl in Book_language.objects.filter(empty_f):
    bl.other_title = bl.book.title
    bl.save()