python - Sorting related items in a Django template

Question

Is it possible to sort a set of related items in a DJango template?

That is: this code (with HTML tags omitted for clarity):

{% for event in eventsCollection %}
   {{ event.location }}
   {% for attendee in event.attendee_set.all %}
     {{ attendee.first_name }} {{ attendee.last_name }}
   {% endfor %}
 {% endfor %}

displays almost exactly want I want. The only thing I want to change is I the list of attendees to be sorted by last name. I've tried saying something like this:

{% for event in events %}
   {{ event.location }}
   {% for attendee in event.attendee_set.order_by__last_name %}
     {{ attendee.first_name }} {{ attendee.last_name }}
   {% endfor %}
 {% endfor %}

Alas, the above syntax doesn't work (it produces an empty list) and neither does any other variation I have thought of (lot's of syntax errors reported, but no joy).

I could, of course, produce some kind of array of sorted attendee lists in my view, but that is an ugly and fragile (and did I mention ugly) solution.

Needless to say, but I'll say it anyway, I have perused the on-line docs and searched Stack Overflow and the archives of django-user without finding anything helpful (ah, if only a query set were a dictionary dictsort would do the job, but it's not and it doesn't)

==============================================

Edited to add additional thoughts after accepting Tawmas's answer.

Tawmas addressed the issue exactly as I presented it -- although the solution was not what I expected. As a result I learned a useful technique that can be used in other situations as well.

Tom's answer proposed an approach I had already mentioned in my OP and tentatively rejected as being "ugly".

The "ugly" was a gut reaction, and I wanted to clarify what was wrong with it. In doing so I realized that the reason it was an ugly approach was because I was hung up on the idea of passing a query set to the template to be rendered. If I relax that requirement, there is an un-ugly approach that should work.

I haven't tried this yet, but suppose that rather than passing the queryset, the view code iterated through the query set producing a list of Events, then decorated each Event with a query set for the corresponding attendees which WAS sorted (or filtered, or whatever) in the desired way. Something like so:

eventCollection = []   
events = Event.object.[filtered and sorted to taste]
for event in events:
   event.attendee_list = event.attendee_set.[filtered and sorted to taste]
   eventCollection.append(event)

Now the template becomes:

{% for event in events %}
   {{ event.location }}
   {% for attendee in event.attendee_list %}
     {{ attendee.first_name }} {{ attendee.last_name }}
   {% endfor %}
 {% endfor %}

The downside is the view has to "actualize" all of the events at once which could be a problem if there were large numbers of events. Of course one could add pagination, but that complicates the view considerably.

The upside is the "prepare the data to be displayed" code is in the view where it belongs letting the template focus on formatting the data provided by the view for display. This is right and proper.

So my plan is to use Tawmas' technique for large tables and the above technique for small tables, with the definition of large and small left to the reader (grin.)

tawmas tawmas · Accepted Answer · 2011-06-30T19:34:09

You need to specify the ordering in the attendee model, like this. For example (assuming your model class is named Attendee):

class Attendee(models.Model):
    class Meta:
        ordering = ['last_name']

See the manual for further reference.

EDIT. Another solution is to add a property to your Event model, that you can access from your template:

class Event(models.Model):
# ...
@property
def sorted_attendee_set(self):
    return self.attendee_set.order_by('last_name')

You could define more of these as you need them...

python - Sorting related items in a Django template

4 Answers