claude-engineering-plugin/plugins/compound-engineering/skills/andrew-kane-gem-writer/SKILL.md

---
name: andrew-kane-gem-writer
description: Write Ruby gems following Andrew Kane's proven patterns and philosophy. Use when creating new Ruby gems, refactoring existing gems, designing gem APIs, or when the user wants clean, minimal, production-ready Ruby library code. Triggers on requests like "create a gem", "write a Ruby library", "design a gem API", or mentions of Andrew Kane's style.
---

# Andrew Kane Gem Writer

Write Ruby gems following Andrew Kane's battle-tested patterns from 100+ gems with 374M+ downloads (Searchkick, PgHero, Chartkick, Strong Migrations, Lockbox, Ahoy, Blazer, Groupdate, Neighbor, Blind Index).

## Core Philosophy

**Simplicity over cleverness.** Zero or minimal dependencies. Explicit code over metaprogramming. Rails integration without Rails coupling. Every pattern serves production use cases.

## Entry Point Structure

Every gem follows this exact pattern in `lib/gemname.rb`:

```ruby
# 1. Dependencies (stdlib preferred)
require "forwardable"

# 2. Internal modules
require_relative "gemname/model"
require_relative "gemname/version"

# 3. Conditional Rails (CRITICAL - never require Rails directly)
require_relative "gemname/railtie" if defined?(Rails)

# 4. Module with config and errors
module GemName
  class Error < StandardError; end
  class InvalidConfigError < Error; end

  class << self
    attr_accessor :timeout, :logger
    attr_writer :client
  end

  self.timeout = 10  # Defaults set immediately
end
```

## Class Macro DSL Pattern

The signature Kane pattern—single method call configures everything:

```ruby
# Usage
class Product < ApplicationRecord
  searchkick word_start: [:name]
end

# Implementation
module GemName
  module Model
    def gemname(**options)
      unknown = options.keys - KNOWN_KEYWORDS
      raise ArgumentError, "unknown keywords: #{unknown.join(", ")}" if unknown.any?

      mod = Module.new
      mod.module_eval do
        define_method :some_method do
          # implementation
        end unless method_defined?(:some_method)
      end
      include mod

      class_eval do
        cattr_reader :gemname_options, instance_reader: false
        class_variable_set :@@gemname_options, options.dup
      end
    end
  end
end
```

## Rails Integration

**Always use `ActiveSupport.on_load`—never require Rails gems directly:**

```ruby
# WRONG
require "active_record"
ActiveRecord::Base.include(MyGem::Model)

# CORRECT
ActiveSupport.on_load(:active_record) do
  extend GemName::Model
end

# Use prepend for behavior modification
ActiveSupport.on_load(:active_record) do
  ActiveRecord::Migration.prepend(GemName::Migration)
end
```

## Configuration Pattern

Use `class << self` with `attr_accessor`, not Configuration objects:

```ruby
module GemName
  class << self
    attr_accessor :timeout, :logger
    attr_writer :master_key
  end

  def self.master_key
    @master_key ||= ENV["GEMNAME_MASTER_KEY"]
  end

  self.timeout = 10
  self.logger = nil
end
```

## Error Handling

Simple hierarchy with informative messages:

```ruby
module GemName
  class Error < StandardError; end
  class ConfigError < Error; end
  class ValidationError < Error; end
end

# Validate early with ArgumentError
def initialize(key:)
  raise ArgumentError, "Key must be 32 bytes" unless key&.bytesize == 32
end
```

## Testing (Minitest Only)

```ruby
# test/test_helper.rb
require "bundler/setup"
Bundler.require(:default)
require "minitest/autorun"
require "minitest/pride"

# test/model_test.rb
class ModelTest < Minitest::Test
  def test_basic_functionality
    assert_equal expected, actual
  end
end
```

## Gemspec Pattern

Zero runtime dependencies when possible:

```ruby
Gem::Specification.new do |spec|
  spec.name = "gemname"
  spec.version = GemName::VERSION
  spec.required_ruby_version = ">= 3.1"
  spec.files = Dir["*.{md,txt}", "{lib}/**/*"]
  spec.require_path = "lib"
  # NO add_dependency lines - dev deps go in Gemfile
end
```

## Anti-Patterns to Avoid

- `method_missing` (use `define_method` instead)
- Configuration objects (use class accessors)
- `@@class_variables` (use `class << self`)
- Requiring Rails gems directly
- Many runtime dependencies
- Committing Gemfile.lock in gems
- RSpec (use Minitest)
- Heavy DSLs (prefer explicit Ruby)

## Reference Files

For deeper patterns, see:
- **[references/module-organization.md](references/module-organization.md)** - Directory layouts, method decomposition
- **[references/rails-integration.md](references/rails-integration.md)** - Railtie, Engine, on_load patterns
- **[references/database-adapters.md](references/database-adapters.md)** - Multi-database support patterns
- **[references/testing-patterns.md](references/testing-patterns.md)** - Multi-version testing, CI setup
- **[references/resources.md](references/resources.md)** - Links to Kane's repos and articles