SimpleCov Test Coverage Agent

Overview

Maintain high test coverage in Ruby and Rails applications through automated analysis using SimpleCov as the coverage engine and SimpleCov Console for terminal output. This skill identifies coverage gaps, suggests targeted tests, and enforces quality standards alongside RubyCritic for comprehensive code quality feedback.

Core Capabilities

1. Setup and Configuration

Configure SimpleCov for any Ruby/Rails project with best practices:

Initial Setup:

# Add to Gemfile
echo "gem 'simplecov', require: false, group: :test" >> Gemfile
echo "gem 'simplecov-console', require: false, group: :test" >> Gemfile
bundle install

Create .simplecov Configuration:

SimpleCov.start 'rails' do
  formatter SimpleCov::Formatter::MultiFormatter.new([
    SimpleCov::Formatter::HTMLFormatter,
    SimpleCov::Formatter::Console
  ])

  # Enable branch coverage (Ruby 2.5+)
  enable_coverage :branch
  primary_coverage :branch

  # Set thresholds
  minimum_coverage line: 90, branch: 80
  minimum_coverage_by_file 80
  refuse_coverage_drop :line, :branch

  # Standard Rails filters
  add_filter '/test/'
  add_filter '/spec/'
  add_filter '/config/'
  add_filter '/vendor/'

  # Organize by application layers
  add_group 'Controllers', 'app/controllers'
  add_group 'Models', 'app/models'
  add_group 'Services', 'app/services'
  add_group 'Jobs', 'app/jobs'
  add_group 'Mailers', 'app/mailers'
  add_group 'Helpers', 'app/helpers'
  add_group 'Libraries', 'lib'
end

Console Formatter Options:

# Customize output in .simplecov
SimpleCov::Formatter::Console.use_colors = true
SimpleCov::Formatter::Console.sort = 'coverage'  # or 'path'
SimpleCov::Formatter::Console.show_covered = false
SimpleCov::Formatter::Console.max_rows = 15
SimpleCov::Formatter::Console.output_style = 'table'  # or 'block'

Test Helper Integration (CRITICAL - Must be FIRST):

# test/test_helper.rb or spec/spec_helper.rb
require 'simplecov'
SimpleCov.start 'rails'

# Now load application
ENV['RAILS_ENV'] ||= 'test'
require_relative '../config/environment'
# ... rest of test helper

2. Running Coverage Analysis

Standard Test Execution:

# Minitest
bundle exec rake test

# RSpec
bundle exec rspec

# Cucumber
bundle exec cucumber

# Specific test files
bundle exec ruby -Itest test/models/user_test.rb

SimpleCov automatically tracks coverage and generates reports after test completion.

Console Output Example:

COVERAGE: 82.34% -- 2345/2848 lines in 111 files
BRANCH COVERAGE: 78.50% -- 157/200 branches

showing bottom (worst) 15 of 69 files
+----------+----------------------------------------------+-------+--------+----------------------+
| coverage | file                                         | lines | missed | missing              |
+----------+----------------------------------------------+-------+--------+----------------------+
| 22.73%   | lib/websocket_server.rb                      | 22    | 17     | 11, 14, 17-18, 20-22 |
| 30.77%   | app/models/role.rb                           | 13    | 9      | 28-34, 36-37         |
| 42.86%   | lib/mail_handler.rb                          | 14    | 8      | 6-8, 12-15, 22       |
| 45.00%   | app/services/payment_processor.rb            | 80    | 44     | 15-22, 35-48, ...    |
+----------+----------------------------------------------+-------+--------+----------------------+

42 file(s) with 100% coverage not shown

HTML Report:

# Open detailed browser report
open coverage/index.html  # macOS
xdg-open coverage/index.html  # Linux

3. Identifying and Addressing Coverage Gaps

Gap Analysis Workflow:

1. Locate worst coverage files from console output

2. Examine specific uncovered lines

3. Categorize gap types:

   • Edge cases and error conditions
   • Branch paths (if/else, case/when)
   • Private methods not exercised through public API
   • Callback sequences
   • Complex conditionals

4. Determine appropriate test type:

   • Unit tests: Business logic, calculations, validations
   • Integration tests: Multi-object workflows
   • System tests: Full user interactions
   • Request/controller tests: HTTP endpoints

5. Write targeted tests

6. Verify improvement

Example: Improving Payment Processor Coverage

SimpleCov shows: 45.00% | app/services/payment_processor.rb | 80 | 44

# View the file with line numbers
cat -n app/services/payment_processor.rb | grep -A2 -B2 "15\|16\|17"

Uncovered lines reveal:

• Lines 15-18: Retry logic for failed charges
• Lines 35-40: Refund processing
• Lines 45-48: Webhook handling

Add Comprehensive Tests:

# test/services/payment_processor_test.rb
require 'test_helper'

class PaymentProcessorTest < ActiveSupport::TestCase
  test "retries failed charges up to 3 times" do
    order = orders(:pending)

    # Simulate failures then success
    Stripe::Charge.expects(:create)
      .times(2)
      .raises(Stripe::CardError.new('declined', nil))
    Stripe::Charge.expects(:create)
      .returns(stripe_charge)

    processor = PaymentProcessor.new(order)
    assert processor.charge
    assert_equal 3, processor.attempt_count
  end

  test "processes refunds correctly" do
    order = orders(:paid)

    processor = PaymentProcessor.new(order)
    refund = processor.refund_payment

    assert refund.succeeded?
    assert_equal order.total, refund.amount
    assert_equal 'refunded', order.reload.status
  end

  test "handles webhook events appropriately" do
    event = stripe_events(:charge_succeeded)

    processor = PaymentProcessor.new
    processor.handle_webhook(event)

    order = Order.find_by(stripe_charge_id: event.data.object.id)
    assert_equal 'paid', order.status
  end
end

4. Branch Coverage Analysis

Branch coverage tracks whether both paths of conditionals are tested.

Understanding Branch Reports:

| 72.22% | app/services/discount_calculator.rb | 4 | 1 | branch: 75% | 4 | 1 | 3[else] |

This shows:

• Line coverage: 72.22% (4 lines, 1 missed)
• Branch coverage: 75% (4 branches, 1 missed)
• Missing branch: Line 3's else path

Example Code:

def calculate_discount(order)
  return 0 if order.total < 50  # Branch: true/false

  discount = order.total * 0.1
  discount > 10 ? 10 : discount  # Branch: true/false
end

Complete Branch Coverage:

test "returns 0 for small orders" do
  order = Order.new(total: 30)
  assert_equal 0, DiscountCalculator.calculate_discount(order)  # Tests true branch line 2
end

test "returns percentage discount for medium orders" do
  order = Order.new(total: 75)
  assert_equal 7.5, DiscountCalculator.calculate_discount(order)  # Tests false branch line 2, false branch line 5
end

test "caps discount at maximum" do
  order = Order.new(total: 200)
  assert_equal 10, DiscountCalculator.calculate_discount(order)  # Tests true branch line 5
end

5. Multi-Suite Coverage Merging

SimpleCov automatically merges results from multiple test suites run within the merge_timeout (default 10 minutes).

Configuration:

# .simplecov
SimpleCov.start 'rails' do
  merge_timeout 3600  # 1 hour

  # Optional: explicit command names
  command_name "Test Suite #{ENV['TEST_ENV_NUMBER'] || Process.pid}"
end

Running Multiple Suites:

# Run all test types - SimpleCov merges automatically
bundle exec rake test
bundle exec rspec
bundle exec cucumber

# View combined coverage
open coverage/index.html

Parallel Test Support:

# test/test_helper.rb
require 'simplecov'

SimpleCov.start 'rails' do
  command_name "Test #{ENV['TEST_ENV_NUMBER']}"
end

bundle exec parallel_test test/ -n 4

6. CI/CD Integration

GitHub Actions:

# .github/workflows/test.yml
name: Tests with Coverage

on: [push, pull_request]

jobs:
  test:
    runs-on: ubuntu-latest

    steps:
      - uses: actions/checkout@v3

      - name: Setup Ruby
        uses: ruby/setup-ruby@v1
        with:
          ruby-version: 3.2
          bundler-cache: true

      - name: Setup Database
        run: |
          bundle exec rails db:create
          bundle exec rails db:schema:load

      - name: Run tests with coverage
        run: bundle exec rake test

      - name: Check coverage thresholds
        run: |
          if [ $? -ne 0 ]; then
            echo "❌ Coverage below threshold"
            exit 1
          fi

      - name: Upload coverage artifacts
        uses: actions/upload-artifact@v3
        if: always()
        with:
          name: coverage-report
          path: coverage/

CI-Specific Configuration:

# .simplecov
SimpleCov.start 'rails' do
  if ENV['CI']
    # Use console formatter only in CI
    formatter SimpleCov::Formatter::Console

    # Strict enforcement
    minimum_coverage line: 90, branch: 80
    refuse_coverage_drop :line, :branch

    # More aggressive thresholds
    minimum_coverage_by_file 85
  else
    # Development: HTML + Console
    formatter SimpleCov::Formatter::MultiFormatter.new([
      SimpleCov::Formatter::HTMLFormatter,
      SimpleCov::Formatter::Console
    ])
  end
end

7. Integration with RubyCritic

Combine SimpleCov with RubyCritic for comprehensive code quality analysis:

Combined Analysis:

# 1. Run tests with coverage
bundle exec rake test

# 2. Run RubyCritic
bundle exec rubycritic app lib --format console

# 3. Review both reports
open coverage/index.html
open tmp/rubycritic/overview.html

Prioritization Matrix:

Complexity	Coverage	Priority	Action
High	Low	CRITICAL	Add tests + refactor
High	High	High	Refactor with test safety net
Low	Low	Medium	Add tests
Low	High	Low	Well-maintained

Example Combined Analysis:

SimpleCov: 45% coverage | app/services/order_processor.rb | 80 lines | 44 missed
RubyCritic: Score D | Complexity 25 | Churn 15

Interpretation:
- High complexity (25) indicates difficult to understand/maintain
- High churn (15) shows frequent changes
- Low coverage (45%) means changes are risky

Action Plan:
1. Write characterization tests for current behavior (increase coverage to ~70%)
2. Refactor to reduce complexity while tests protect against regression
3. Achieve 90%+ coverage on simplified code
4. Monitor churn - frequent changes may indicate unclear requirements

Advanced Features

Conditional Coverage (On-Demand)

Run coverage only when explicitly requested:

# test/test_helper.rb
SimpleCov.start if ENV['COVERAGE']

# Without coverage
bundle exec rake test

# With coverage
COVERAGE=true bundle exec rake test

Nocov Exclusions

Exclude specific code sections:

# :nocov:
def debugging_helper
  # Development-only code not covered
  puts "Debug: #{inspect}"
end
# :nocov:

# Custom token
SimpleCov.nocov_token 'skip_coverage'

# skip_coverage
def skip_this_method
end
# skip_coverage

Custom Filters and Groups

Advanced Filtering:

SimpleCov.start 'rails' do
  # Exclude short files
  add_filter do |source_file|
    source_file.lines.count < 5
  end

  # Exclude files by complexity
  add_filter do |source_file|
    # Could integrate complexity metrics
    source_file.lines.count > 500
  end

  # Array of filters
  add_filter ["/test/", "/spec/", "/config/"]
end

Custom Groups:

SimpleCov.start 'rails' do
  add_group "Services", "app/services"
  add_group "Jobs", "app/jobs"
  add_group "API", "app/controllers/api"

  add_group "Long Files" do |src_file|
    src_file.lines.count > 100
  end

  add_group "Business Logic" do |src_file|
    src_file.filename =~ /(models|services|lib)/
  end
end

Subprocess Coverage

Track coverage in forked processes:

SimpleCov.enable_for_subprocesses true

SimpleCov.at_fork do |pid|
  SimpleCov.command_name "#{SimpleCov.command_name} (subprocess: #{pid})"
  SimpleCov.print_error_status = false
  SimpleCov.formatter SimpleCov::Formatter::SimpleFormatter
  SimpleCov.minimum_coverage 0
  SimpleCov.start
end

Coverage for Spawned Processes

For processes started with PTY.spawn, Open3.popen, etc:

# .simplecov_spawn.rb
require 'simplecov'
SimpleCov.command_name 'spawn'
SimpleCov.at_fork.call(Process.pid)
SimpleCov.start

# In test
PTY.spawn('ruby -r./.simplecov_spawn my_script.rb') do
  # ...
end

Troubleshooting

Coverage Shows 0% or Missing Files

Problem: SimpleCov doesn't track files or shows 0%.

Cause: SimpleCov loaded after application code.

Solution: Ensure SimpleCov starts FIRST:

# ✅ CORRECT
require 'simplecov'
SimpleCov.start 'rails'
require_relative '../config/environment'

# ❌ WRONG
require_relative '../config/environment'
require 'simplecov'
SimpleCov.start

Spring Conflicts

Problem: Inaccurate coverage with Spring.

Solutions:

# Option 1: Eager load
require 'simplecov'
SimpleCov.start 'rails'
Rails.application.eager_load!

# Option 2: Disable Spring for coverage
# DISABLE_SPRING=1 bundle exec rake test

# Option 3: Remove Spring
# Remove gem 'spring' from Gemfile

Parallel Test Conflicts

Problem: Results overwrite each other.

Solution:

SimpleCov.start 'rails' do
  command_name "Test #{ENV['TEST_ENV_NUMBER'] || Process.pid}"
end

Branch Coverage Not Showing

Problem: Branch coverage is 0% or missing.

Requirements:

• Ruby 2.5 or later
• Must explicitly enable

Solution:

SimpleCov.start do
  enable_coverage :branch
  primary_coverage :branch
end

Old Cached Results

Problem: Coverage seems incorrect or stale.

Solution:

# Clear cache
rm -rf coverage/
bundle exec rake test

# Or increase merge timeout
SimpleCov.merge_timeout 7200  # 2 hours

Best Practices

1. Start Early

Set up SimpleCov at project inception to establish baselines and track progress from day one.

2. Set Achievable Thresholds

Start with realistic targets (80-85%) and increase gradually. Avoid demanding 100% immediately.

3. Track Both Line and Branch Coverage

Branch coverage reveals untested conditional paths that line coverage misses.

minimum_coverage line: 90, branch: 80

4. Prioritize Business Logic

Focus coverage efforts on:

• Domain models
• Service objects
• Complex calculations
• Critical user flows

Less critical:

• View helpers
• Configuration files
• Simple CRUD controllers

5. Make Coverage Part of Code Review

Include coverage reports in PR reviews. Block PRs that drop coverage without justification.

refuse_coverage_drop :line, :branch

6. Don't Chase 100% Blindly

Focus on meaningful tests over coverage percentage. Some code (error logging, debugging helpers) may not need testing.

7. Use Appropriate Grouping

Organize reports by architecture to identify weak layers:

add_group "Domain Models", "app/models"
add_group "Business Logic", "app/services"
add_group "Background Jobs", "app/jobs"
add_group "API", "app/controllers/api"

8. Filter Wisely

Exclude generated code, migrations, and test infrastructure:

add_filter '/db/migrate/'
add_filter '/test/'
add_filter '/config/initializers/'
add_filter '/vendor/'

9. Merge All Test Suites

Ensure coverage reflects complete test suite execution:

bundle exec rake test      # Unit/integration
bundle exec rspec          # Specs
bundle exec cucumber       # Features
# SimpleCov merges automatically

10. Enforce in CI/CD

Prevent coverage degradation by failing builds:

if ENV['CI']
  minimum_coverage line: 90, branch: 80
  refuse_coverage_drop :line, :branch
end

Common Patterns

Pre-commit Hook

#!/bin/bash
# .git/hooks/pre-commit

echo "Running tests with coverage..."
COVERAGE=true bundle exec rake test

if [ $? -ne 0 ]; then
  echo "❌ Coverage check failed"
  exit 1
fi

echo "✅ Coverage acceptable"

Coverage Summary Script

# scripts/coverage_summary.rb
require 'json'

data = JSON.parse(File.read('coverage/.resultset.json'))
coverage = data.values.first.dig('coverage', 'lines')

total = coverage.size
covered = coverage.compact.count { |x| x > 0 }
pct = (covered.to_f / total * 100).round(2)

puts "Coverage: #{pct}% (#{covered}/#{total} lines)"

exit 1 if pct < 90

Watch Mode for TDD

# Use guard-minitest or guard-rspec
bundle exec guard

# Gemfile
group :development, :test do
  gem 'guard-minitest'
end

# Guardfile
guard :minitest do
  watch(%r{^test/(.*)/?(.*)_test\.rb$})
  watch(%r{^app/(.+)\.rb$}) { |m| "test/#{m[1]}_test.rb" }
end

Resources

Scripts

See scripts/ for coverage analysis utilities (if provided).

References

See references/ for:

• Advanced configuration examples
• CI/CD integration patterns
• Coverage analysis methodologies

External References

• SimpleCov Documentation
• SimpleCov Console Formatter
• Ruby Coverage Library
• RubyCritic Integration