Split queryset or get multiple querysets by field instead of just ordering by field

Question

I want to query a database in a way such that rather than just ordering by some field, I get a separate QuerySet (or dictionary, list, whatever) for each unique value of that field. Hopefully the below example will help:

Suppose a model like

Class Person(models.Model):
   first_name = models.CharField()
   last_name = models.CharField

Calling Person.objects.all().order_by('last_name') gives me a single long QuerySet. I want instead to have a separate list for each unique last_name. So one list for every Person with last_name="Smith" and another list for every Person with last_name="Nguyen" etc.

Obviously I can't know ahead of time what last_names will be in the database, nor how many people will share a common last_name. Is there any fast, efficient, or automatic way to do this in django or do I just need to process the data myself after getting the one large queryset back?

Answer 1

samy's code is correct, but pretty inefficient: you do n+1 queries (where n is the number of unique last names). Since you're going to be getting all the objects in the table anyway, you might as well do it in one go. Use this instead:

from collections import defaultdict
person_dict = defaultdict(list)
persons = Person.objects.all()
for person in persons:
    person_dict[person.last_name].append(person)

Answer 2

Look at the regroup tag.

{% regroup persons by last_name as surname_list %}

<ul>
{% for last_name in surname_list %}
    <li>{{ last_name.grouper }}
    <ul>
        {% for person in last_name.list %}
          <li>{{ person.last_name }}, {{ person.first_name }}</li>
        {% endfor %}
    </ul>
    </li>
{% endfor %}
</ul>

(this code is untested, please read the docs and fix any problem yourself)