Test Coverage & SimpleCov
Coverage metrics help us focus testing effort, highlight dead code, and track regressions. SimpleCov integrates seamlessly with both Minitest and RSpec.
Configure SimpleCov
# Gemfile
group :test do
gem 'simplecov', require: false
end
# test/test_helper.rb (or spec/spec_helper.rb)
require 'simplecov'
SimpleCov.start 'rails' do
enable_coverage :branch
minimum_coverage 90
add_group 'Services', 'app/services'
add_group 'Queries', 'app/queries'
add_group 'Jobs', 'app/jobs'
add_group 'Controllers', 'app/controllers'
end
Use branch coverage to catch missing conditionals. Adjust `minimum_coverage` carefully—enforce attainable targets (e.g., 90%) and build culture around meaningful tests, not gaming metrics.
Interpreting coverage reports
HTML report
Open `coverage/index.html` to see per-file metrics, branch misses, and hotspots. Lines highlighted in red need tests; yellow indicates partial coverage.
Trending metrics
Export coverage summaries as JSON (`SimpleCov::Formatter::JSONFormatter`) and push to Datadog or GitHub Checks for trend tracking in CI.
Strategy for meaningful coverage
- Prioritize business-critical modules (`app/services`, `app/queries`). We group them in SimpleCov to keep the dashboard actionable.
- Exclude auto-generated files (`config/`, `db/schema.rb`, vendor code) to prevent noisy metrics.
- Set a threshold delta in CI—fail builds if coverage drops by more than 0.5% compared to main.
- Use branch coverage to detect missing `else` clauses or guard branches.
Automation tips
- Run coverage on nightly builds to keep feedback fast for local TDD loops; optional in every PR if performance permits.
- Upload coverage from parallel CI runs using `simplecov-cobertura` to merge results.
- Surface coverage failures prominently in PR templates so reviewers can request missing tests before merge.