Rails Engine Compatibility

Core principle: Every claimed Rails/Ruby version must be in the CI matrix. Prefer explicit support targets over accidental compatibility.

HARD-GATE

Before claiming support for a Rails/Ruby version:
  1. bundle exec rake zeitwerk:check        # verify autoloading on each version
  2. bundle exec rspec                       # full suite per matrix version
  3. CI matrix must pass — not just main Rails version

DO NOT ship compatibility changes without verifying both autoloading and full suite.

Quick Reference

Core Checks

1. Define supported Ruby and Rails versions — state them in gemspec and README.
2. Run bundle exec rake zeitwerk:check — file paths must match constant names exactly.
3. Check initializer behavior across boot and reload — use config.to_prepare for reload-sensitive hooks.
4. Verify gemspec dependency bounds match tested versions: spec.add_dependency "rails", ">= 7.0", "< 8.0" — bounds must match what CI actually tests.
5. Check optional integrations (jobs, mailers, assets, routes) per version.
6. CI matrix must run against each claimed Rails/Ruby combination:

strategy:
  matrix:
    include:
      - { ruby: "3.2", rails: "7.1" }
      - { ruby: "3.3", rails: "7.2" }

Pitfalls

Key Example: Feature Detection

# ❌ Bad — brittle, wrong for patch versions
if Rails.version >= "7.0"
  config.active_support.cache_format_version = 7.0
end

# ✅ Good — detect the capability directly
if ActiveSupport::Cache.respond_to?(:format_version=)
  config.active_support.cache_format_version = 7.0
end

If an EXAMPLES.md is present in the repository, consult it for additional patterns covering gemspec bounds, Zeitwerk file/constant naming, reload-safe hooks, and CI matrix YAML. If not, the Pitfalls table and Key Example above cover the most common cases.

Output Style

When asked to improve compatibility:

1. State the support matrix being targeted.
2. List the most likely breakpoints.
3. Make compatibility changes in isolated, testable seams.
4. Recommend matrix coverage if it does not exist.

Integration

Assets

The following supplementary files may be present in the repository. Consult them if available; the skill body above is self-contained if they are not.

• assets/compatibility_matrix.md — version support matrix template
• assets/zeitwerk_notes.md — extended Zeitwerk naming and configuration notes