Doing more with your Django models
So you have a Django app, but sometimes you find the Django models too constraining. We will guide you through using Django models to get more out of them. This is an intermediate tutorial, as some familiarity with Django is assumed. For example, we assume you know how to write a basic Django model, you know how to override Python methods, as well as how .filter
and .exclude
work.
We will talk about these topics
- Proxy Models
- Overriding
.save
- Using signals
- Optimizing your DB access using
.extra
- Advanced lookups using Q objects
- Aggregation and Annotation
- Using F() expressions
Lets look at some common operations you may want to perform using Django and how the above Django functionality will help you achieve them.
How can I get two Python representation of the same Database table?
You may want to have two model classes corresponding to a single database table. For example, admin.site.register
allows a Model to be registered only once. However, you may want the same model twice in the Admin area. Proxy models can help you do that!
from django.contrib.auth.models import User
class NewUser(User):
class Meta:
proxy = True
Now in your admin.py
you can register NewUser again and customize your ModelAdmin. (For example, if you want to show only some of the fields, add a custom ordering and so on).
How can I take action before saving a model to database?
Sometime you may have some denormalized data. Consider this model:
class Poll(models.Model):
###...
num_choices = models.PositiveIntegerField()
class Choice(models.Model):
poll = models.ForeignKey(Poll)
###...
You want to increment the num_choices
before saving Choice
. You can do that by overriding .save
like this.
def save(self, *args, **kwargs):
self.poll.num_choices += 1
self.poll.save()
super(Choice, self).save(*args, **kwargs)
How can I take action before saving the models to database if I didn’t write the model?
Overriding .save
is great when you are writing all the models. However for example you have a Subscription
model and when someone sings up they are assigned a subscription. However since you didn’t write the User
model, you can not override the .save model.
Django emits signals before taking any action. You can connect your functions to signals to take action when interesting stuff happens. Django comes with two signals
pre_save
and post_save
which you can connect to.
from django.db.models.signals import pre_save
from django.contrib.auth.models import User
def subscription_handler(**kwargs):
#Do something with the Subscription model
pre_save.connect(subscription_handler, sender=User, dispatch_uid="subscription_handler")
How can I get related objects without hitting the database many times?
Assume we have these models:
class Subject(models.Model):
###...
class Score(models.Model):
###...
subject = models.ForeignKey(Subject)
score = models.PositiveIntegerField()
Now you are iterating over a Subject queryset, and you want the sum of all the Score objects which have a foreign key to current object. You can do this by getting individual Score
objects and then summing them in Python, but it would be faster to do that in the database. Django has a method .extra
which allows you to insert arbitrary clauses in the sql generated by the queryset. For example here you can do
Subject.objects.extra(select={"total_scores": "select sum(score) from poll_score where poll_score.subject_id = poll_subject.id"})
assuming that the app is called poll
for which the default names for tables are poll_subject
and poll_score
.
How can you compose OR, NOT and other SQL operations?
By default Django will AND all criteria passed to the filtering methods. If you want to use OR/NOT operator, you will need to use Q objects.
We have a model like:
class Score(models.Model):
###...
subject = models.ForeignKey(Subject)
score = models.PositiveIntegerField()
date = models.DateField()
So, if you want all Score objects for Physics which have either score > 95 or are in 2012.
criteria = Q(subject__name="Physics") & (Q(score__gt=95)|Q(date__year=2012))
We used the double underscore notation to apply filters and joined them together using boolean operators. You can pass them to .filter
. (Or to .exclude
)
Score.objects.filter(criteria)
How can I get group_by type of operations?
Django provides two methods on its querysets – .aggregate
and .annotate
. Aggregates convert the queryset in a dictionary on name, value pairs.
E.g., if you want the maximum, minimum, and average of Score
objects. You can get them as
from django.db.models import Avg, Max, Min
Score.objects.all().aggregate(Max('score'), Avg('score'), Min('score'))
For more, see the guide on aggregation
How can I compare within rows?
Django provides F
objects which are used to create queries which compare within rows.
We have a model like this:
class Department(models.Model):
##...
num_employees = models.PositiveIntegerField()
num_managers = models.PositiveIntegerField()
You want to find all departments which have more managers than employees.
from django.db.models import F
Department.objects.filter(num_managers__gt=F('num_employees'))
F objects support addition, subtraction, multiplication, division so you can do things like
Department.objects.filter(num_employees__lt=F('num_managers')*2)