filter prefetch_related and select_related results

Question

I have 3 models

models.py

class Book(models.Model):
     name = models.CharField(max_length=255, primary_key=True)
     ...

class BookTranslation(models.Model):
     book = models.ForeignKey(Book, related_name='translations')
     language = models.CharField(max_length=2, choices=LANGUAGE_CHOICES)
     ...

class Chapter(models.Model):
     book_translation=models.ForeignKey(BookTranslation,related_name='chapters')
     ...

I would like to create a DetailView for a book in just one language, my first approach was something like this:

views.py

class BookDetail(DetailView):
    model = Book

    def get_object(self):
        return self.model.objects.select_related('translations').prefetch_related('translations__chapters').get(slug=self.kwargs['slug'], translations__language=self.kwargs['language'])

But this way i return all related BookTranslations and Chapters, not only those in the language i want...
Is it possible to filter the select_related so that in my template, in {{book.translations}} i only have the translation in the language i asked for (same for chapters)?

Answer 1

Since you want to Get a Book with a specific BookTranslation, so its better that you base your Query on BookTranslation and get the Book object from it.

i.e. Use something like this

class BookDetail(DetailView):
    model = Book

    def get_object(self):
        self.booktranslation = BookTranslation.objects.select_related('book').get(book__slug=self.kwargs['slug'],
                                     language=self.kwargs['language'])
        return self.booktranslation.book

You can then pass self.booktranslation as a context variable to access that in template.

Answer 2

You can try something like this to hit the database only once and fetch the related results:

class BookDetail(DetailView):
    model = Book

    def get_object(self):
        return self.model.objects.filter(slug=self.kwargs['slug'],
                                         translations__language=self.kwargs['language'],
                                         translations__chapter__name=self.kwargs['ch_name'], ) \
                                 .values('translations__language',
                                         'translations__another_field', 
                                         'translations__chapter__name',
                                         'translations__chapter__another_field', )[0]