Django models referencing each other

April 19, 2022

I have two models and I want to put an if condition in my template to get the desired output.
I have the first model

class Subject(models.Model):
    name = models.CharField(max_length=50)
    books = models.ForeignKey('Books')

And the second one

class Books(models.Model):
    name = models.CharField(max_length=50)
    subject = models.ForeignKey(Subject, on_delete=models.CASCADE)

And a view

def View(request):
    subjects = Subject.objects.all()
    return render('my_html.html",{'subjects':subjects})

In my model, I have wanted to do this.

{% for subject in subjects %}
    {% if subject.books.count > 0 %}
        {{ subject.name }}
    {% endif %}
{% endfor %}

I thought the only way is to have the models referencing each other for my plan to work but I also think that looping will require more resources. Is there a better way of doing this either thro’ the view or just in the models?

>Solution :

You only need to ForeignKey from Book to Subject, so:

class Subject(models.Model):
    name = models.CharField(max_length=50)
    # no books

class Books(models.Model):
    name = models.CharField(max_length=50)
    subject = models.ForeignKey(Subject, on_delete=models.CASCADE)

then you can filter the subjects such that it only retrieves Subjects with at least one book:

def view(request):
    subjects = Subject.objects.filter(books__isnull=False).distinct()
    return render(request, 'my_html.html', {'subjects': subjects})

this will make a LEFT OUTER JOIN on the table of the Book model, and check if there is at least one book. You thus do not need to filter in the template (which is not a good idea anyway).