A left outer reverse select_related in Django?

django left-join django-queryset django-select-related
augustomen picture augustomen · Jun 4, 2010 · Viewed 10.5k times · Source

Imagine the following model:

class Parent(Model):
    ...

class Child(Model)
    father = ForeignKey(Parent)
    ...

Some parents have children, others do not (they're not parents in the real meaning, it's just a fictional name).

I would like to make the following query: I want to list all the Parents, and if they have children, bring me the children too. That would be the equivalent of a left outer join to Child table, that is:

select * from app_parent left join app_child on child_father_id=parent_id

This way, when I invoke Parent.child_set in my template, I won't hit the database a gazillion times. Is there a way to do that? Thanks

Answer

Gautier Hayoun picture Gautier Hayoun · Jun 30, 2012

Starting from Django 1.4 prefetch_related does what you want.

Parent.objects.prefetch_related('child_set')

Related(!) django docs : https://docs.djangoproject.com/en/dev/ref/models/querysets/#prefetch-related.

Related questions

Django ForeignKey with null=True, inner join, and left outer join

Let's say I have two Django models Person and Company as follows: - class Company(models.Model): name = models.CharField() class Person(models.Model): last_name = models.CharField(blank=True) first_name = models.CharField() company = models.ForeignKey(Company, null=True, …

Read More
Django ORM. Joining subquery

I have a table which contains list of some web sites and a table with statistics of them. class Site(models.Model): domain_name = models.CharField( max_length=256, unique=True, ) class Stats(models.Model): date = models.DateField() site = models.ForeignKey(…

Read More
'pip' is not recognized as an internal or external command

I'm running into a weird error when trying to install Django on my computer. This is the sequence that I typed into my command line: C:\Python34> python get-pip.py Requirement already up-to-date: pip in c:\python34\lib\site-packages …

Read More