Here's something nobody tells you when you're building your Rails app: that beautiful, working codebase you shipped last year? It's already aging. Not in a nostalgic, fine-wine kind of way, but more like milk left on the counter.
And before you know it, you're staring at deprecation warnings, incompatible gems, and a security audit that makes you want to hide under your desk.
Since 2006, we at RailsFactory have walked through the upgrade trenches with more than 50 Rails upgrades. Some were small startups running on Rails 4, others were massive platforms handling millions of users. Each one taught us something new about what goes wrong, what breaks first, and how to fix it before anyone notices.
So if you're putting off that Rails upgrade because it feels overwhelming, you're not alone. But here's the thing: it doesn't have to be a nightmare. Let me show you exactly what to expect and how to handle it.
Why Upgrades Go Sideways
Let's be honest. Most teams delay Rails upgrades because they're afraid of breaking things. And that fear is completely justified. We've seen it happen. A perfectly functional app suddenly throws 500 errors after a seemingly innocent version bump. Background jobs stop processing. User uploads fail. The CEO starts getting support tickets.
But here's the reality check: staying on an old version isn't the safe choice anymore. One crowdfunding platform we worked with upgraded from Rails 4.2 to 6.1 and saw their app availability jump to 96%. Before the upgrade, they were dealing with frequent downtime and performance issues that frustrated their user base.
The security angle is even scarier. One application running on Rails 4 had 34 vulnerabilities in the Docker image alone. After moving to Rails 7? Just 2 low-level vulnerabilities remained. That's not a small improvement. That's the difference between getting hacked and sleeping soundly at night.
Read the full case study here: Modernizing Rails application for a leading crowdfunding platform
The Usual Suspects: What Actually Breaks
After 50+ upgrades, we've noticed patterns. The same issues show up repeatedly, regardless of the application size or industry. Here are the biggest troublemakers:
1. Gem Dependencies Turn Into Dependency Hell
This one tops the list every single time. Your Gemfile is basically a house of cards, and upgrading Rails is like pulling out the bottom card while praying everything else stays upright.
Some gems simply aren't ready for newer Rails versions. You'll get errors during bundle update, or worse, methods that worked perfectly fine suddenly go missing at runtime. We've seen popular gems that haven't been updated in years, abandoned by their maintainers, sitting there like landmines in your codebase.
The solution? You need a rails compatibility checker before you even think about running that upgrade command. We built RailsUp specifically for this reason.
Upload your Gemfile, and it scans every single dependency, flags the ones that won't play nice with your target Rails version, and gives you a complexity score plus estimated upgrade time. No surprises, no guesswork.
We worked with an immigration platform that relied heavily on rack-throttle-maintenance, which was no longer actively maintained. That single gem became a blocker for their entire upgrade path. We had to find alternatives, test them thoroughly, and ensure the replacement provided the same functionality without breaking existing features.
Read the full case study here: Successful Rails 7 Upgrade: Slashed Security Risks by 90% for an Immigration Platform
2. Autoloading Changes Break Your Carefully Organized Code
Rails changed how it loads code between versions, particularly with the introduction of Zeitwerk. If you've got a custom folder structure or monkey patches living in lib, you're in for a fun time.
Suddenly, constants that loaded perfectly fine stop working. Classes can't find each other. Your app boots, but then crashes mysteriously when users hit specific routes. We've spent hours debugging these issues because the error messages are often cryptic.
The pattern we see most often? Teams put service objects, custom validators, or utility classes in non-standard directories, and Rails no longer knows where to find them. You'll need to either move these files to places Zeitwerk expects or configure autoload paths explicitly.
3. Configuration Files Multiply Like Rabbits
Every major Rails version brings new configuration options. After an upgrade, you'll notice files like
new_framework_defaults_6_0.rb
or
new_framework_defaults_7_0.rb
appearing in your config folder.
These files contain all the new Rails defaults, but they're commented out by default to prevent breaking your app immediately. The problem? Many developers forget about them entirely. Then, months later, they wonder why their app isn't taking advantage of Rails 7's performance improvements or security enhancements.
You need to go through these files methodically, understand what each setting does, and enable them one by one. Test after each change. Otherwise, you're not really upgrading, you're just running new Rails code with old Rails behavior.
4. Asset Pipeline and JavaScript Tooling Become a Puzzle
Remember Webpacker? Rails 6 loved it. Rails 7? Not so much. Rails 7 drops Webpacker in favor of import maps, esbuild, or alternative JavaScript bundlers. If your app has a complex frontend with lots of npm packages, this transition will test your patience. You'll need to decide: do you stick with a JavaScript bundler (and which one?), or do you embrace import maps and potentially refactor your frontend setup?
We've seen teams spend weeks on this alone. The JavaScript ecosystem moves fast, and Rails is trying to keep up while also simplifying things. But that simplification means relearning how to handle assets.
5. Database Migrations Have Hidden Landmines
Database changes between Rails versions can be subtle but dangerous. Column types behave differently. Indexes get created with new naming conventions. Foreign key constraints that worked before suddenly throw errors.
One sales software platform we upgraded from Rails 4 to 7.2 required careful, incremental migration handling. We couldn't just run all migrations at once because that would lock tables and cause downtime for their users who were actively making sales.
The trick is running migrations in staging first, checking query performance, and ensuring they won't lock tables for hours in production. Use tools to analyze migration safety before deployment.
Read the full case study here: Sales Software Modernization: Rails 7 Upgrade
6. Background Jobs Start Acting Strange
Sidekiq, Delayed Job, whatever you're using, background jobs often break in weird ways during upgrades. Serialization formats change. Job arguments that worked fine suddenly can't be deserialized. Jobs that were enqueued before the upgrade fail after deployment.
This one bites you at the worst possible time: right after you've deployed to production and everyone's celebrating. Then the monitoring alerts start firing because your job queue is backing up.
Always test your background job infrastructure separately. Deploy to a staging environment, enqueue jobs, then upgrade, and verify those jobs still process correctly. And have a rollback plan ready.
7. Test Suite Explosions
You'd think tests would catch problems, right? Sometimes they do. Sometimes they just break in spectacular fashion, making it impossible to tell what's actually wrong with your code versus what's wrong with the test setup.
RSpec matchers change behavior. Factory Bot (formerly FactoryGirl) has breaking changes between versions. Capybara's syntax evolves. Your test database setup might need adjustments.
We worked with a marketing automation platform upgrade that had no comprehensive testing framework, making it extremely tough to ensure reliability during upgrades. Every change was a leap of faith. After the upgrade, we helped them build out proper test coverage so future upgrades wouldn't be so terrifying.
If your test coverage is below 80%, honestly, fix that before upgrading. You're flying blind otherwise.
8. Security Vulnerabilities Hiding in Plain Sight
One of the platforms we upgraded was using very old versions of Ruby and Rails with many known security issues. These weren't theoretical vulnerabilities. They were documented exploits that attackers actively target.
The scarier part?
Many teams don't realize how exposed they are until they run security audits. Those CVEs aren't abstract, they represent real attack vectors into your application and user data.
Modern rails upgrade services include security audits as part of the process. You want to know exactly what vulnerabilities you're closing by upgrading and what new security features you're gaining access to.
The Infrastructure Surprises Nobody Expects
Upgrading Rails doesn't happen in isolation. Your infrastructure feels it too.
A sales software application saw CPU utilization drop from 70-90% down to 20-30% after upgrading from Rails 4 to Rails 7. Memory usage also stabilized between 30-40% even during peak hours. These performance improvements let them downsize from servers with 16 vCPUs and 32GB RAM to just 8 vCPUs and 16GB RAM.
That's real money saved every month, not to mention faster response times for users.
But getting there requires infrastructure adjustments:
Docker images need updates - Your base images might not support the new Ruby version. You'll need to rebuild containers, update your Dockerfile, and test the whole deployment pipeline.
Environment variables and secrets management changes - We migrated one client's sensitive information to AWS Secret Manager for improved security during their Rails upgrade. This wasn't strictly required by Rails, but it made sense to modernize everything at once.
CI/CD pipelines break - Your automated tests run on specific Ruby and Rails versions. When you upgrade, your entire continuous integration setup needs adjustment. We've seen deployments fail because the CI environment had the wrong Ruby version installed.
Database connection pooling behaves differently - Rails 6 and 7 have different defaults for database connections. If you don't adjust these settings, you might hit connection limits in production that never showed up in staging.
The Smart Way to Approach Upgrades
Here's what we've learned works consistently across all those 50+ projects:
Never skip versions
The golden rule is upgrading one major version at a time: 6.0 to 6.1 to 7.0. Skipping versions creates gem incompatibilities and missing migrations that are nightmares to debug.
Use a rails upgrade tool to plan everything first
Before touching any code, use rails compatibility checker tools like RailsUp to assess your current situation. Get that complexity score. Understand which gems need replacement. Know your estimated timeline. Plan the upgrade like a project, not a quick task
Set up dual booting early
This lets you run your app on both the old and new Rails versions simultaneously. You can verify behavior matches, catch regressions early, and make the final switch with confidence.
Communicate with everyone involved
Share timelines, assign ownership, outline rollback strategies, and define branching rules. The more visibility your team has, the smoother everything goes. We've seen upgrades derail because developers worked in silos and stepped on each other's changes.
Schedule strategically
Schedule the upgrade outside of major product launches, marketing pushes, or critical business cycles. Give your team breathing room to test, roll back if needed, and redeploy without pressure.
Test with real data patterns
Staging environments with dummy data don't cut it. Use anonymized production data volumes and real user behavior patterns. We've found bugs that only appeared with specific data combinations that never existed in our test fixtures.
The Hidden Benefits That Make It Worthwhile
Yes, upgrades are work. But they're not just defensive moves to avoid security problems. The benefits go further than you might think.
Developer happiness improves dramatically
Your team stops fighting with outdated syntax and missing features. They can use modern Ruby patterns. They can install current gems without compatibility juggling. Recruiting becomes easier because developers want to work with recent technology, not maintain legacy systems.
Feature development speeds up
Once you're on a current Rails version, adding new features becomes faster. You have access to better debugging tools, improved logging, and performance optimizations built into the framework.
Your codebase becomes cleaner naturally
Upgrades force you to confront technical debt. Those workarounds for old Rails bugs? You can remove them. That monkey patch from 2016? Probably unnecessary now. The cleanup happens organically as part of the upgrade process.
Deployment confidence grows
After successfully navigating a Rails upgrade, your team gains valuable experience with systematic code changes, comprehensive testing, and production deployments. This confidence carries over to other projects.
When to Call in Backup
Look, not every team has the bandwidth or expertise to handle Rails upgrades internally. We've worked with companies where the development team is slammed with feature requests, and carving out three months for an upgrade just isn't realistic.
That's okay. Specialized rails upgrade services exist exactly for this scenario. We handle everything from the initial audit through production deployment and post-upgrade monitoring. Your team keeps shipping features while we manage the technical migration.
The key is recognizing when you need help:
- If your Rails version is three or more major versions behind
- If you don't have comprehensive test coverage
- If your team lacks experience with Rails upgrades
- If you're dealing with a massive application with complex dependencies
- If downtime isn't acceptable for your business
Professional rails upgrade services aren't about admitting defeat. They're about making smart resource allocation decisions and getting expert help for a specialized task.
Your Next Steps
If you've made it this far, you're probably sitting on an aging Rails app and wondering what to do next. Here's my advice:
Start by assessing where you stand. Run a rails compatibility checker like RailsUp on your codebase today. Get that complexity score. See which gems are going to cause problems. Understand what you're facing.
Then make a plan. You can use a pre-upgrade checklist. If the complexity is low and your team has capacity, tackle it internally. If it's complex or your team is stretched thin, reach out for help. Either way, stop putting it off.
We've upgraded everything from tiny side projects to applications handling millions of requests per day. The process gets easier when you know what to expect and have the right tools at hand.
The apps we upgraded years ago? They're still running strong, secure, and performant. The teams maintaining them are happier because they're working with modern tools instead of fighting legacy constraints.
Your Rails app deserves the same treatment. And honestly, your developers do too.
Ready to start planning your upgrade? The hardest part is making the decision. Everything else is just execution, testing, and deployment. And we've done that fifty times over.
