Django - Problem with DB query using foreign key - django

Question

I'm using Django for a Web app and I have the following data model:
class classi(models.Model):
nome = models.TextField(null=True)
class Meta:
db_table = 'classi'
class users(models.Model):
name = models.TextField(null=True)
email = models.TextField(null=True)
password = models.TextField(null=True)
classe = models.ForeignKey(classi, db_column='classe', on_delete=models.CASCADE, null=True)
class Meta:
db_table = 'users'
class smartphone(models.Model):
marca = models.TextField(null=True)
modello = models.TextField(null=True)
possessore = models.ForeignKey(users, db_column='possessore', on_delete=models.CASCADE, null=True)
class Meta:
db_table = 'smartphone'
My goal is to show, on an HTML page, all classi, and for each classi all users and for each user all smartphone.
How can I implement my view.py and my html file?
The only solution that I found is to scan all table with a for loop and, through a condition, select the row using foreign key:
{% for c in classi %}
<p>{{ c.nome }}</p>
{% for u in users %}
{% if u.classe == c %}
<p>{{ u.name }}, {{ u.email }}, {{ u.password }}</p>
{% for s in smartphone %}
{% if s.possessore == u %}<p>{{ s.marca }}, {{ s.modello }}</p> {% endif %}
{% endfor %}
{% endif %}
{% endfor %}
{% endfor %}
Is there a better solution?

Answer 1

You could use backwards relations, which will only have items that have a connection
The general format is obj.{model}_set but you can also set it to something different in the models.py with the related_name attr in the ForeignKey field
{% for c in classi %}
<p>{{ c.nome }}</p>
{% for u in c.users_set.all %}
<p>{{ u.name }}, {{ u.email }}, {{ u.password }}</p>
{% for s in u.smartphone_set.all %}
<p>{{ s.marca }}, {{ s.modello }}</p>
{% endfor %}
{% endfor %}
{% endfor %}
Or, and this is probably cleaner:
Go the reverse way by looping through the smartphones in the top and just order smartphones by nome in the view with smartphoneList = smartphone.objects.all().order_by('possessore__classe__nome') so they are grouped together
{% for s in smartphoneList %}
<p>{{ s.possessore.classe.nome }}</p>
<p>{{ s.possessore.name }}, {{ s.possessore.email }}, {{ s.possessore.password }}</p>
<p>{{ s.marca }}, {{ s.modello }}</p>
{% endfor %}
Note after writing this I noticed #Lucas Grugru as already posted the reverse relations way.. From my testing it doesn't require pre-fetching, but it's probably a good idea to do that (I also have no idea). I'm still posting this because I think looping through smartphones might be a cleaner way of doing it

Answer 2

You can use *prefetch_related for prefetching related object from foreign key. More information: https://docs.djangoproject.com/fr/4.1/ref/models/querysets/#prefetch-related
The view:
def view(request):
classe = Classi.objects.all().prefetch_related("users_set").prefetch_related("users_set__smartphone_set")
context = {}
context["classe"] =) classe
return render(request, 'template.html', context)
And the html:
{% for c in classe %}
<p>{{ c.nome }}</p>
{% for u in c.users_set.all %}
<p>{{ u.name }}, {{ u.email }}, {{ u.password }}</p>
{% for s in u.smartphone_set.all %}
<p>{{ s.marca }}, {{ s.modello }}</p>
{% endfor %}
{% endfor %}
{% endfor %}