CODEWITHSUNDEEP
djangodjango-modelsreverse-lookup
yetty
yetty

Reputation: 2466

Django - reverse lookups

For example, I have these models:

class Person(models.Model):
    name = models.CharField(max_length=20)
    employer = models.CharField(max_length=20)

class Car(models.Model):
    person = models.ForeignKey(Person)
    name = models.CharField(max_length=10)
    model = models.CharField(max_length=10)
    ...

I want to get all people who own some specific car:

people = Person.objects.filter(car__name="Toyota")

Now I want to write these people out with details about their own car. I can do this:

for person in people:
   ...
   cars = person.car_set.filter(name="Toyota")
   ...

But it hit the database again. How can I avoid this? Is there any way to do this simpler?

Upvotes: 2

Views: 2733

Answers (3)

arustgi
arustgi

Reputation: 848

First select the car and the related person when the car name

cars = Car.objects.select_related("person").filter(name="Toyota").order_by("person")

Now you have all the cars whose name is toyota along with the person for that car, ordered_by person.

Now use the python itertools.groupby to group this list for each person

from itertools import groupby
for k, g in groupby(cars, lambda x: x.person):
       person = k
       cars = list(g)

Now in that iteration you have the person and his cars (whose name is "toyota"). You'll notice that there is only single query that occurs, and then the following operations execute on the cached information.

Upvotes: 3

Zach
Zach

Reputation: 19082

I don't believe there is anyway to avoid the additional database hits. If you're concerned about making too many queries, you can try something like this:

from collections import defaultdict

cars = Car.objects.filter(**kwargs).selected_related('person')
owners = defaultdict(list)

for car in cars:
    owners[car.person].append(car)

This should only be one query that selects all the relevant cars and the data about their related person

Upvotes: 0

kcbanner
kcbanner

Reputation: 4078

Check out select_related(), I've used it before to turn many small queries that span multiple models into one large query: https://docs.djangoproject.com/en/dev/ref/models/querysets/#select-related

It works by pre-populating the QuerySet, so your access to car_set will already be there and won't result in a new query.

Upvotes: 0

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