Migrations introduction

Migrations are version-controlled schema changes—like Git for your database structure. Django generates them from model changes and applies them with migrate.

Workflow

1. Edit models.py
2. python manage.py makemigrations — creates migration files
3. python manage.py migrate — applies to database

Best practices

• Commit migration files to git—teams must share the same history
• Review auto-generated migrations before deploying
• Never edit applied migrations in production—create new ones
• Data migrations use RunPython for backfills

Important interview questions and answers

1. Q: makemigrations vs migrate?
   A: makemigrations writes Python migration files from model diffs; migrate executes them against the DB.
2. Q: Squashing migrations?
   A: Combines many migrations into one for cleaner history in long projects—plan carefully.
3. Q: Zero-downtime deploys?
   A: Add nullable columns first, backfill, then enforce NOT NULL in a follow-up migration.

Self-check

1. Which command creates migration files?
2. Why commit migrations to version control?

Tip: Run makemigrations after every model change and commit the files—never "fix" production by hand-editing tables without a migration.