Question

I have a small probleme concerning django querysets and select_related.

With this model :

class DeviceGroup(models.Model):
  name = models.CharField(max_length=255, unique=True)
  owner = models.ForeignKey(User)

class Device(models.Model):
  name = models.CharField(max_length=255)
  address = models.GenericIPAddressField()
  port = models.IntegerField()
  group =    models.ForeignKey(DeviceGroup)

class Sensor(models.Model):
  device = models.ForeignKey(Device)

I’d like to get a list like :

- group 1
  - device 1
  - device 2
  - device 3
    -sensor 1
- group 2
- group 3
  - device 4
  - device 5
    - sensor 2

I’ve read the doc about select_related(), but I don’t know how to do because I’ve to start my query bu group cause, it’s directly linked to logged user.

Answer 1

If you want to present it this way in a Django template, you probably want to use regroup:
https://docs.djangoproject.com/en/dev/ref/templates/builtins/#regroup

The new docs is slightly more obfuscate, I find the old docs version easy to comprehend (this eg from https://docs.djangoproject.com/en/1.3/ref/templates/builtins/#regroup ):

{% regroup people by gender as gender_list %}

<ul>
{% for gender in gender_list %}
    <li>{{ gender.grouper }}
    <ul>
        {% for item in gender.list %}
        <li>{{ item.first_name }} {{ item.last_name }}</li>
        {% endfor %}
    </ul>
    </li>
{% endfor %}
</ul>

Where in your case:

from view.py

group = Group.objects.all()

template.html

{% regroup groups by device as device_list %}