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

you can get all the unique lastnames:

from django.db.models import Count
...
last_names = Person.objects.values('last_name').annotate(Count('last_name')) # This will return all the unique last_names
values = dict( ((last_name['last_name'], Person.objects.all().filter(last_name = last_name['last_name'])) for last_name in last_names if last_name['last_name__count']) )
# This will create a dictionary where the keys are all the unique names and the values are querysect of all the values that have that lastname

Daniel-Roseman is right it is pretty inefficient so heres a tweak version …

from collections import defaultdict
values = defaultdict(list)
_ = map(lambda person: values[person.last_name].append(person), Person.objects.all())

note that _ = ... is so we don’t print all the None on the terminal 😉