Iterating over results, sorted by a m2m - django

Question

My data model is simple:
class Neighborhood(models.Model):
name = models.CharField(max_length = 50)
slug = models.SlugField()
class Location(models.Model):
company = models.ForeignKey(Company)
alt_name = models.CharField()
neighborhoods = models.ManyToManyField(Neighborhood)
I would like to supply a page on my site that lists all locations by their neighborhood (s). If it were singular, I think {% regroup %} with a {% ifchanged %} applied to the neighborhood name would be all that I need, but in my case, having it be a m2m, I'm not sure how do this. A location may have multiple neighborhoods, and so I would like them to be redundantly displayed under each matching neighborhood.
I'm also aware of FOO_set but that's per Object; I want to load the entire data set.
The final result (in the template) should be something like:
Alameda
Crazy Thai
Castro
Kellys Burgers
Pizza Orgasmica
Filmore
Kellys Burgers
Some Jazz Bar
Mission
Crazy Thai
Elixir
...
The template syntax would (ideally?) look something like:
{% for neighborhood in neighborhood_list %}
{% ifchanged %}{{ neighborhood.name }}{% endifchanged %}
{% for location in neighborhood.restaurants.all %}
{{ location.name }}
{% endfor %}
{% endfor %}

Answer 1

I'd just do it the expensive way and cache the result over scratching my head. Your template example would work fine until performance becomes an issue in generating that one page per X cache timeout.
You could do it in python as well if the result set is small enough:
# untested code - don't have your models
from collections import defaultdict
results = defaultdict(list)
for location_m2m in Location.neighborhoods.through.objects.all() \ # line wrap
.select_related('neighborhood', 'location', 'location__company__name'):
if location_m2m.location not in results[location_m2m.neighborhood]:
results[location_m2m.neighborhood].append(location_m2m.location)
# sort now unique locations per neighborhood
map(lambda x: x.sort(key=lambda x: x.location.company.name), results.values())
# sort neighborhoods by name
sorted_results = sorted(results.items(), key=lambda x:x[0].name)

Answer 2

create another model that would define your m2m relationship:
class ThroughModel(models.Model):
location = models.ForeignKey('Location')
neighborhood = models.ForeignKey('Neighborhood')
and use it as the through model for your m2m field:
class Location(models.Model):
...
neighborhoods = models.ManyToManyField(Neighborhood, through='ThroughModel')
Then you can get all your neighbourhoods sorted:
ThroughModel.objects.select_related().order_by('neighborhood__name')
This is untested.
Or, if you cannot change the database, just do a raw SQL join.