How to implement such an if, for construct in an html file?

Question

Django

There are two models Book and Author (the codes will be below). You need to write this: if the Author has a Book, then each Book will be displayed in the list. There is a Many-to-One relationship between Book and Author:

class Book(models.Model):
    title = models.CharField(max_length=200)
    author = models.ForeignKey('Author', on_delete=models.SET_NULL, null=True)
    summary = models.TextField(max_length=1000, help_text="Enter a brief description of the book")
    isbn = models.CharField('ISBN', max_length=13, help_text='13 Character <a href="https://www.isbn-international.org/content/what-isbn">ISBN number</a>')
    genre = models.ManyToManyField(Genre, help_text="Select a genre for this book")
    language = models.ForeignKey('Language',  on_delete=models.SET_NULL, null=True)
    
    def display_genre(self):
        return ', '.join([genre.name for genre in self.genre.all()[:3]])
    display_genre.short_description = 'Genre'
    
    def __str__(self):
        return self.title

    def get_absolute_url(self):
        return reverse('book-detail', args=[str(self.id)])


class Author(models.Model):
    """
    Model representing an author.
    """
    first_name = models.CharField(max_length=100)
    last_name = models.CharField(max_length=100)
    date_of_birth = models.DateField(null=True, blank=True)
    date_of_death = models.DateField('Died', null=True, blank=True)
    
    def get_absolute_url(self):
        """
        Returns the url to access a particular author instance.
        """
        return reverse('author-detail', args=[str(self.id)])
    

    def __str__(self):
        """
        String for representing the Model object.
        """
        return '%s, %s' % (self.last_name, self.first_name)

Answer 1

models.py:

class Book(models.Model):
    ...
    author = models.ForeignKey(Author, related_name="books", on_delete=models.SET_NULL, null=True)
    ...

views.py:

def index(request):
    context = {}
    authors = Author.objects.all()
    context['authors'] = authors
    return render(request, 'index.html', context)

index.html:

...
    <div>
        {% for author in authors %}
            <h1>{{ author.first_name }}</h1>
            {% for book in author.books.all %}
                <h4>{{ book.title }}</h4>
                {% empty %}
                    <p>Похоже, у этого автора нет книг :(</p>
            {% endfor %}
        {% endfor %}
    </div>
...

And the author model must be declared before the book model.
I also advise you to read this: https://docs.djangoproject.com/en/3.0/topics/db/qu...