Django Interview Questions
Django interviews tend to cluster around a handful of recurring themes: how the ORM generates SQL and where N+1 query problems sneak in, how a request flows through URL routing, middleware, and views before producing a response, and how Django REST Framework layers serialization and authentication on top of that core request cycle. Strong candidates aren't just able to recite definitions — they can explain the tradeoffs, like when select_related beats prefetch_related, or why a particular middleware ordering matters, with a concrete example.
Cricket analogy: It's like a cricket commentary panel not just naming the shot played (a cover drive) but explaining why the batsman chose it against that particular bowler's line — interviewers want the reasoning, not just the term.
ORM and Models Questions
The single most common ORM interview question is explaining the N+1 query problem: iterating over a queryset and accessing a related object (post.author.name) inside the loop triggers one query per iteration unless you've pre-fetched it. select_related solves this for forward foreign key and one-to-one relationships by generating a SQL JOIN in a single query, while prefetch_related solves it for reverse foreign keys and many-to-many relationships by issuing a second query and joining the results in Python, since those relationships can't be flattened into one SQL JOIN without duplicating rows.
Cricket analogy: The N+1 problem is like a scorer looking up each batsman's career average individually from a filing cabinet after every single delivery, instead of pulling the whole squad's stats sheet once before the innings starts.
# N+1 problem: one query per post to fetch author
for post in Post.objects.all():
print(post.author.name) # extra query each iteration
# Fixed with select_related (forward FK -> SQL JOIN, 1 query)
for post in Post.objects.select_related("author"):
print(post.author.name)
# Fixed with prefetch_related (reverse FK/M2M -> 2 queries, joined in Python)
for post in Post.objects.prefetch_related("comments"):
print(post.comments.count())
Request/Response Cycle Questions
Interviewers often probe whether you understand that middleware runs in the order defined in MIDDLEWARE on the way in (request phase) and in reverse order on the way out (response phase), which matters for things like SecurityMiddleware needing to run before SessionMiddleware can rely on the session being set up correctly. Another common question is explaining Django's URL resolution: an incoming request's path is matched against urlpatterns (checked in order, first match wins), which resolves to a view function or class-based View.as_view(), which then returns an HttpResponse that middleware can still modify before it goes back to the client.
Cricket analogy: Middleware order is like fielding positions being set before the ball is bowled (request phase) and then the field adjusting after the shot is played (response phase) — the sequence in which each fielder reacts matters.
A favorite follow-up question: 'What's the difference between a function-based view and a class-based view?' Function-based views are explicit and easy to trace top-to-bottom; class-based views (like ListView, DetailView) reduce boilerplate for common CRUD patterns through inheritance and mixins, at the cost of needing to understand the MRO (method resolution order) when things go wrong.
Django REST Framework and Architecture Questions
For teams building APIs, expect questions on Django REST Framework's core building blocks: serializers convert querysets/model instances to JSON (and validate incoming JSON back into Python data), while ViewSets combined with a Router auto-generate the standard list/create/retrieve/update/delete URL patterns from one class. A common deeper question is explaining permission_classes versus authentication_classes: authentication determines *who* the request claims to be (e.g., via a token or session), while permissions determine *what* that identified (or anonymous) user is allowed to do, and both run before your view's business logic executes.
Cricket analogy: Authentication versus permission is like a stadium checking your ticket to confirm who you are (authentication) versus checking whether that ticket class lets you into the players' pavilion (permission) — two separate checks.
A common trap in interviews: candidates confuse authentication failures (401 Unauthorized — 'I don't know who you are') with permission failures (403 Forbidden — 'I know who you are, but you can't do this'). DRF returns these distinctly, and mixing them up in an API design answer is a red flag for interviewers.
- The N+1 query problem and its fixes (select_related for forward FK/O2O, prefetch_related for reverse FK/M2M) are the most common ORM interview topic.
- Middleware runs in MIDDLEWARE order on the request phase and reverse order on the response phase.
- URL resolution checks urlpatterns in order and stops at the first match, dispatching to a view or class-based View.as_view().
- Function-based views are explicit; class-based views reduce CRUD boilerplate via inheritance and mixins.
- DRF serializers handle both serialization to JSON and validation of incoming data back into Python objects.
- Authentication identifies who the request claims to be; permissions determine what that identity is allowed to do — both run before view logic.
- 401 means unauthenticated; 403 means authenticated but not permitted — mixing these up is a common interview red flag.
Practice what you learned
1. What causes the N+1 query problem in Django?
2. When should you use select_related instead of prefetch_related?
3. In what order does middleware run during the response phase?
4. What is the difference between authentication and permissions in DRF?
5. What HTTP status code typically indicates an authentication failure versus a permission failure?
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