Database Migrations and Application Code: A Common Developer Mistake

As developers begin their journey in software development, one of the most common mistakes is bundling database migrations with application code changes in the same pull request. While it might seem logical to keep related changes together, this practice can lead to serious problems.

Let's explore why this happens and how to develop better habits early in your career.

Why Do Developers Make This Mistake?

The "Keep Related Changes Together" Trap

Many developers think: "These changes are related, so they should go in the same PR!" This intuition seems logical but doesn't account for deployment complexities.

Lack of Deployment Experience

Without experiencing the pain of failed deployments, it's hard to understand why separation matters. New developers often learn this lesson the hard way.

# A typical beginner's pull request might look like this:
# PR #123 - Add user preferences feature

# migration.py
def migrate():
    execute_sql("ALTER TABLE users ADD COLUMN preferences JSONB")

# user_model.py
def update_preferences(self, new_prefs):
    self.preferences = new_prefs  # What happens if migration fails?
    self.save()        

Real-World Horror Stories

The "It Works on My Machine" Scenario

Developer: "I tested it locally and everything works!" Production: Migration fails halfway through deployment. Result: Application crashes because code expects new column Recovery Time: Several stressful hours

The Rollback Nightmare

Timeline: 1. Deploy combined changes. 2. Users report bugs 3. Try to rollback 4. Database changes already affected production data 5. Can't rollback cleanly(What if we just wanted to revert the code not the migration) 6. Team needs emergency fixes

Learning Better Practices

Step 1: Separate Your Concerns

Instead of one small/big PR, create two:

PR #1 - Database Migration
- Add preferences column
- Ensure backward compatibility
- Can be deployed safely

PR #2 - Application Code
- Update user model
- Add new features
- References successful migration        

Step 2: Safe Migration Pattern

-- PR #1: Database Changes
ALTER TABLE users ADD COLUMN preferences JSONB DEFAULT '{}';
-- Notice the DEFAULT value for backward compatibility        

# PR #2: Application Changes (submitted after PR #1 is deployed)
class User:
    def get_preferences(self):
        return self.preferences or {}        

Common Early Career Questions

"But it's just a small change!"

Even small changes can cause big problems. Size isn't the issue - it's about safe deployment practices.

"It's faster to do it all at once!"

Actually, fixing problems from combined PRs takes much longer than doing it right the first time.

"My team does it this way..."

This is a great opportunity to introduce better practices to your team, backed by solid reasoning.

Building Good Habits Early

• Could this migration fail?
• What happens if we need to rollback?
• Is this backward compatible?

Learning from Senior Developers

Senior developers often share these insights:

• "Migrations are forever, code is temporary"
• "You can always rollback code, but data changes persist"
• "Better to take an extra hour now than spend a night fixing production"

Conclusion

I have done all this mistake in my career and learn this lesson the hard way.

Mistakes is part of learning, but some mistakes are easier to avoid with proper guidance. Separating database migrations from application code changes is a fundamental best practice that every developer should learn early. It might seem like extra work at first, but it's an investment in reliability and maintainability that pays off immediately.

Remember:

• Separate concerns
• Think about deployment safety
• Build good habits early
• Learn from other's experiences

Your future self (and your team) will thank you for taking the time to do things right.

I would suggest everyone to read more on:

• Database migration patterns
• Deployment best practices
• Code review guidelines
• Production incident case studies