Django Fundamentals: MVT Architecture, the ORM, and Middleware

Common Mistakes Worth Remembering

• The N+1 query problem — by far the most common Django performance bug, and an easy one to introduce without noticing since each query individually looks fine.
• Forgetting to run makemigrations after a model change, then wondering why migrate has nothing to apply.
• Putting business logic in templates instead of views — templates should stay close to presentation-only.
• Using Model.objects.get() when a matching row might not exist, without catching DoesNotExist — an unhandled 500 instead of a clean 404.
• Defaulting every ForeignKey to on_delete=CASCADE without considering whether cascading deletes are actually the intended behavior for that relationship.
• Forgetting {% csrf_token %} on a POST form, then being confused by a 403.
• Placing custom middleware in the wrong order — e.g., before the SessionMiddleware or AuthenticationMiddleware it actually depends on.
• Overusing signals for logic that would be clearer, and easier to trace, living directly in a view or model method.

Python's language-level fundamentals (the ones that show up independent of any framework) are covered in Python Fundamentals for Interviews. Practice querysets, views, and the N+1 fix as runnable exercises in the code lab.