Counting all items linked to foreign key - django

Question

I'm looking for a solution to combine in one view information from two models.
One model is for "node" definition. Second model is "drive" definition, where I have foreign key for "node". One "node" can contain many drives.
In a template I need to show all assigned drives for a given node in a list view. I'm interested in count, a number of drives assigned to given node.
I don't have any new idea how calculate this information and pass to template. Should I count this as a "pure python"? I believe there is a way in Django as this doesn't look very complex.
View:
class NodeListView(ListView):
template_name = 'nodes/nodes_list.html'
queryset = Node.objects.all()
context_object_name = 'nodes'
def get_context_data(self, **kwargs):
context = super().get_context_data(**kwargs)
drives = Drive.objects.all()
context.update({
'drives': drives
})
return context
Models:
class Node(models.Model):
name = models.CharField(max_length=32)
.... more ....
class Drive(models.Model):
node = models.ForeignKey(Node, related_name='nodes', on_delete=models.CASCADE)
.... more ....

Answer 1

You can annotate the Nodes with the number of related Drives:
from django.db.models import Count
class NodeListView(ListView):
template_name = 'nodes/nodes_list.html'
queryset = Node.objects.annotate(ndrives=Count('nodes'))
context_object_name = 'nodes'
then you can render this with:
{% for node in nodes %}
{{ node.ndrives }}
{% endfor %}
You can also enumerate over the related Drives with:
{% for node in nodes %}
{{ node.ndrives }}
{% for drive in node.nodes.all %}
{{ drive }}
{% endfor %}
{% endfor %}
where you can boost efficiency with:
from django.db.models import Count
class NodeListView(ListView):
template_name = 'nodes/nodes_list.html'
queryset = Node.objects.annotate(
ndrives=Count('nodes')
).prefetch_related('nodes')
context_object_name = 'nodes'
The reason that this is named nodes is because of the related_name='nodes'. This however does not make much sense: related_name='drives' would make more sense, since this deals with a collection of Drives, you thus might want to rename this to:
class Node(models.Model):
name = models.CharField(max_length=32)
# …
class Drive(models.Model):
node = models.ForeignKey(
Node,
related_name='drives',
on_delete=models.CASCADE
)
# …
Then the view thus looks like:
from django.db.models import Count
class NodeListView(ListView):
template_name = 'nodes/nodes_list.html'
queryset = Node.objects.annotate(
ndrives=Count('drives')
).prefetch_related('drives')
context_object_name = 'nodes'
and we render this with:
{% for node in nodes %}
{{ node.ndrives }}
{% for drive in node.drives.all %}
{{ drive }}
{% endfor %}
{% endfor %}