1 · Structure and rendering · 30 MIN

Django models and database constraints

Database constraints protect invariants across application processes.

A model defines persistent fields and indexes. A unique slug is enforced by the database, so two concurrent writers cannot both create the same value successfully. Create migration files and apply them before querying a new schema. QuerySets are lazy: constructing one does not immediately fetch its rows. Restricting selected fields and page size prevents accidentally loading an entire large table. The model stores public course metadata; user progress should use a separate relation with an ownership key.

Apply migrations to a disposable development database.

Read the example

# courses/models.py
from django.db import models
class Course(models.Model):
    slug = models.SlugField(unique=True)
    title = models.CharField(max_length=160)
    published = models.BooleanField(default=False, db_index=True)
    class Meta:
        ordering = ["id"]
# Run: python manage.py makemigrations courses
# Then: python manage.py migrate
# In python manage.py shell:
# Course.objects.create(slug="react", title="React foundations", published=True)
# list(Course.objects.filter(published=True).values("slug", "title")[:20])

After migration and insertion, the query returns the published React course. A duplicate slug violates the unique constraint.

Your challenge

Add a lesson model related to Course, migrate it and query a course with its ordered lessons without an N+1 loop.

Solution cost: Depends on indexes, selected rows and joins; inspect the query plan. time · O(p) for p materialized page rows. space

Common trap

Calling list on an unbounded QuerySet materializes every matching row.

# courses/models.py
from django.db import models
class Course(models.Model):
    slug = models.SlugField(unique=True)
    title = models.CharField(max_length=160)
    published = models.BooleanField(default=False, db_index=True)
    class Meta:
        ordering = ["id"]
# Run: python manage.py makemigrations courses
# Then: python manage.py migrate
# In python manage.py shell:
# Course.objects.create(slug="react", title="React foundations", published=True)
# list(Course.objects.filter(published=True).values("slug", "title")[:20])

Further reading: Official documentation