Why you need to use Subqueries in Django

The Django ORM is a powerful tool but certain aspects of it are counterintuitive, such as the SQL order of execution. Let’s look at an example of this trap and how we can fix it using subqueries: class Book(models.Model): class Meta: constraints = [ models.UniqueConstraint( fields=["name", "edition"], name="%(app_label)s_%(class)s_unique_name_edition", ) ] name = models.CharField(max_length=255) edition = models.CharField(max_length=255) release_year = models.PositiveIntegerField(null=True) I want to write a query that reads: Out of the latest books, give me the ones with a non-null release year....

July 21, 2021 · 2 min · Johnny Metz

Check your Django Migrations on every commit

Keeping your models in sync with your migrations is an important part of any Django app. My team and I frequently make changes to our models and we occassionally forget to create new migrations for those changes. This results in errors and data loss. Let’s look at an easy way to ensure your models and migrations are always in sync: We’ll use a simple Product model. class Product(models.Model): name = models....

May 8, 2021 · 2 min · Johnny Metz

Find all N+1 violations in your Django app

The N+1 problem is a common database performance issue. It plagues ORM’s, such as Django and SQLAlchemy, because it leads to your application making more database queries than necessary. Let’s look at a basic example in Django. class Artist(models.Model): name = models.CharField(max_length=255) class Song(models.Model): artist = models.ForeignKey(Artist, on_delete=models.CASCADE) name = models.CharField(max_length=255) def print_songs(): for song in Song.objects.all(): print(f"{song.artist.name}- {song.name}") Now let’s create a unit test to ensure print_songs runs successfully. We’re using pytest and pytest-django so we can create the test data using a pytest fixture....

April 13, 2021 · 3 min · Johnny Metz