Upgrading from Rails 5 to Rails 6 brings meaningful improvements like Zeitwerk autoloading, Action Mailbox, Action Text, parallel testing, and multiple database support. But for most teams, the biggest headache during the upgrade isn't the framework itself. It's the gems.
Your application likely depends on dozens, if not hundreds, of gems, and not all of them keep pace with Rails releases. This guide walks through five of the most common gem compatibility issues developers face when moving from Rails 5 to Rails 6, along with practical fixes for each one.Businesses planning a smooth migration often work with an experienced Ruby on Rails development company to minimize compatibility risks and reduce downtime.
Why Gem Compatibility Matters During a Rails Upgrade
Rails 6 introduced several under the hood changes, including a new autoloading mechanism (Zeitwerk), updated Bundler requirements, and changes to how Rails handles asset management. Any gem that relies on the old autoloading behavior, monkey patches internal Rails classes, or hasn't been updated in a while can break your application in subtle and frustrating ways.
Identifying these issues early, before they show up in production, saves significant debugging time. Below are the five compatibility problems that come up most often, and how experienced Rails developers resolve them.
1. Zeitwerk Autoloading Conflicts
Rails 6 switched the default autoloader from the classic loader to Zeitwerk. Zeitwerk enforces stricter naming conventions between file paths and constant names, which means gems or internal code that don't follow standard Rails naming conventions can suddenly fail to load.
Common symptoms:
- NameError: uninitialized constant errors that never appeared under Rails 5
- Classes or modules defined with non standard file names
- Namespacing issues where folder structure doesn't match module nesting
How to fix it:
Run bin/rails zeitwerk:check to identify autoloading problems before they surface at runtime. Rename files and folders so they exactly match the class or module names they define. If a specific gem is the source of the issue, check whether the maintainers have released a Zeitwerk-compatible version. As a temporary workaround, you can fall back to the classic autoloader in config/application.rb, though this should be treated as a short-term fix rather than a long-term solution
2. Bundler and Gemfile.lock Version Mismatches
Rails 6 requires a newer version of Bundler than many Rails 5 applications were using. This often leads to Gemfile.lock conflicts, especially in teams where different developers have different Bundler versions installed locally.
Common symptoms:
- Bundler could not find compatible versions errors
- CI pipelines failing even though the app runs fine locally
- Gemfile.lock constantly changing between commits
How to fix it:
Update Bundler across your team and CI environment to a version that supports Rails 6, then regenerate your Gemfile.lock with bundle update --bundler. Commit the updated lock file so everyone, including your CI pipeline, is working from the same dependency graph. It also helps to pin your Bundler version explicitly in the Gemfile to avoid drift going forward.
3. Sprockets and Webpacker Conflicts
Rails 6 shifted toward Webpacker as the default JavaScript bundler, while many Rails 5 apps still rely heavily on Sprockets and the asset pipeline. Gems that inject JavaScript or CSS through Sprockets, such as older UI component libraries, can conflict with Webpacker or simply stop loading assets correctly.
Common symptoms:
- Missing stylesheets or JavaScript after deployment
- Duplicate asset compilation errors
- Gems that expect //= require style directives no longer working as expected
How to fix it:
Decide early whether you want to keep Sprockets, migrate fully to Webpacker, or run both side by side, which Rails 6 supports. For gems that only ship Sprockets-compatible assets, check for a Webpacker-friendly alternative or manually import the required JavaScript through app/javascript/packs. Keeping your asset management strategy consistent across your team avoids unnecessary confusion.
4. Deprecated or Abandoned Gems
Some gems that worked fine on Rails 5 are simply no longer maintained. This is one of the most common blockers in real-world upgrades, especially with older gems like protected_attributes, paperclip, or custom authentication gems built against outdated Rails internals.
Common symptoms:
- Gems that monkey patch ActiveRecord or ActionController internals in ways Rails 6 no longer supports
- Deprecation warnings that turn into hard errors
- No new releases from the gem maintainer in years
How to fix it:
Audit your Gemfile before starting the upgrade and flag any gem that hasn't been updated recently. For deprecated gems, look for actively maintained replacements. For example, Paperclip users typically migrate to Active Storage, which ships with Rails itself. If no replacement exists and the gem is small, consider forking it and applying the necessary compatibility patches yourself.
5. ActionCable and WebSocket Related Gem Issues
Rails 6 made changes to how Action Cable handles connections and configuration, which can cause compatibility problems with gems that extend or hook into WebSocket functionality, particularly those used for real time notifications or live updates.
Common symptoms:
- WebSocket connections dropping unexpectedly
- Configuration options that silently stop working
- Errors related to connection adapters, especially with Redis based setups
How to fix it:
Review the changelog of any Action Cable-related gem before upgrading and confirm it explicitly supports Rails 6. Double-check your cable.yml configuration, since default adapter settings changed between versions. If you're using Redis as your Action Cable backend, verify that your Redis gem version is also compatible with the updated Action Cable requirements.
A Practical Checklist Before You Upgrade
To make the process smoother, consider following these steps before touching your production environment:
- Run bundle outdated to see which gems are behind
- Run bundle outdated to see which gems are behind.
- Check each gem's changelog or GitHub issues for Rails 6 support.
- Upgrade Bundler and regenerate your lock file.
- Run zeitwerk:check to catch autoloading issues early.
- Test asset compilation in a staging environment.
- Upgrade one major gem category at a time rather than all at once.
Final Thoughts
Upgrading from Rails 5 to Rails 6 is a worthwhile investment, but gem compatibility issues are often the biggest source of friction. Autoloading changes, Bundler version mismatches, asset pipeline conflicts, abandoned gems, and Action Cable updates account for the majority of problems teams encounter.
By auditing your dependencies early, testing incrementally, and replacing unmaintained gems where necessary, you can move through the upgrade with far fewer surprises and set your application up for a smoother path to future Rails versions as well.
If you're looking for a reliable software development company to handle complex Rails migrations, W3villa Technologies offers expertise in upgrading applications while maintaining performance, stability, and long-term scalability.
