Displaying data from intermediate table in a many-to-many relation - django

Question

I have 3 models that follow the principle described in Django documentation (https://docs.djangoproject.com/en/2.2/topics/db/models/#extra-fields-on-many-to-many-relationships):
class Topic(models.Model):
key = models.CharField(max_length=255, primary_key=True)
persons = models.ManyToManyField('Person', through='Interest')
class Person(models.Model):
first_name = models.CharField(max_length=255)
last_name = models.CharField(max_length=255)
interests = models.ManyToManyField('Topic', through='Interest')
class Interest(models.Model):
person = models.ForeignKey(Person, on_delete=models.CASCADE)
topic = models.ForeignKey(Topic, on_delete=models.CASCADE)
source = models.ForeignKey(TopicSource, on_delete=models.CASCADE)
The views.pyis really simple:
class TopicsView(generic.ListView):
template_name = 'app/topics.html'
context_object_name = 'topics_list'
def get_queryset(self):
return Topic.objects.all()
The template is actually giving me headaches:
<table>
<tbody class="list">
{% for item in topics_list %}
<tr>
<td>{{ item.key }}</td>
<td>{{ item.person_set.all.count }}</td>
<td>
<ul>
{% for person in item.person_set.all %}
<li>{{ person.last_name }}, {{ person.first_name }} [{% person.interests_set.get(cluster=item) %}]</li>{% endfor %}
</ul>
</td>
</tr>
{% endfor %}
</tbody>
</table>
With {% person.interests_set.get(topic=item) %}, I'm trying to access data from the intermediate table.
How can I display the source of the interest next to the name of the person?
This solution is giving hint on how to do this from the shell but I cannot figure out how to achieve that in the template.

Answer 1

Django templates don't support what you're trying to do out of the box. You would have to create a custom tag to implement what you're trying.
However you could do:
class TopicsView(generic.ListView):
template_name = 'app/topics.html'
context_object_name = 'topics_list'
queryset = Topic.objects.prefetch_related(
Prefetch(
'interest_set',
Interest.objects.select_related('person')
)
Then adjust your template to iterate over the interests.
<table>
<tbody class="list">
{% for item in topics_list %}
<tr>
<td>{{ item.key }}</td>
<td>{{ item.interest_set.all.count }}</td>
<td>
<ul>
{% for interest in item.interest_set.all %}
<li>{{ interest.person.last_name }}, {{ interest.person.first_name }} [ {{ interest.source }} ]</li>{% endfor %}
</ul>
</td>
</tr>
{% endfor %}
</tbody>
</table>

Answer 2

As an alternative to the solution proposed by #schillingt (the accepted solution), I came up with the below solution:
views.py:
class InterestsView(generic.ListView):
template_name = 'app/interests.html'
context_object_name = 'interests_list'
def get_queryset(self):
return Interest.objects.order_by('topic', 'person').all()
template:
{% regroup interests_list by topic as l1%}
<table>
<tbody>
{% for topic in l1 %}
<tr>
<td>{{ topic.grouper }}</td>
<td>{{ topic.list|length }}</td>
<td>
<ul>
{% for item in topic.list%}
<li>{{ item.person.last_name }}, {{ item.person.first_name }} [{{ item.source }}]</li>
{% endfor %}
</ul>
</td>
</tr>
{% endfor %}
</tbody>
</table>
However, the solution from #schillingt is better since it's performance are a lot better (by a factor 2),