Custom Rails Generators in RubyEvents (Rails World 2026)

At RubyEvents.org, we generate a lot of YAML. Contributors need an easy, idiomatic way to generate event files to the latest standards in an unfamiliar codebase. AI needs to generate files from unstructured data without drifting. These feel like different problems, but custom Rails generators can solve both.

Rails generators power rails generate — a built-in tool for creating and modifying files from the command line. Custom generators feel natural for anyone familiar with Rails and allow you to extend Rails' conventions with your own.

This will be a deep dive into how to build a custom generator — templates, mutating files, testing, and everything we learned the hard way at RubyEvents.

Transcript

1. Custom Rails Generators in RubyEvents Rachael Wright-Munn (@ChaelCodes)
2. Marco Roth Adrien Poly Hans Schnedlitz Rachael (Me) 󰎤 󰏃 󰎈 󰑔 RubyEvents Team
3. We index Events
4. Using Yaml!
5. You are Here.
6. Inherited architectural decision Why do you use yaml? Allows us to review events Allows any developer to update events Historical Record
7. Data changes require commits.
8. I needed a tool that would take unstructured data and turn it into commits.
9. GitHub Copilot
10. Spoiler Alert - This will work!
11. Invented Keys Problem: AI is super-smart, but it doesn't know our system. Used old/deprecated format No # TODO comments Useful keys like `canceled` or `original_title` neglected Spends lots of tokens understanding examples
12. This is true of contributors too!
13. 180+ Contributors!
14. Invented Typo Keys Problem: Contributors are super-smart, but they don't know our system. Used old/deprecated format No # TODO comments Useful keys like `canceled` or `original_title` neglected Spends lots of tokens time understanding examples
15. How do I teach them about RubyEvents?
16. MCP SKILL + CLI AI-native SDK that fetches context or executes commands for a model Markdown documentation discoverable by AI paired with a normal CLI Preferred by AI Usable without tokens Context consumed in describing MCP tool Documentation and Context in Skill Context = Tokens ($) Validations in Skill
17. SKILL + CLI!
18. Rails Generators are a familiar pattern!
19. Arguments and options Why Generators? Templating and file generation Documentation Testing File conflicts
20. Rails didn't have to make these decisions either.
21. I want to make a generator!
22. Generate a Generator
23. cfp_generator
24. USAGE
25. cfp_generator_test
26. How does it name things?
27. cfp_generator
28. application_name = rubyevents class_name = CFP edit_helper = edit_cfp_url(@cfp) NamedBase file_path = cfp fixture_file_name = cfps human_name = CFP i18n_scope = cfp index_helper = cfps
29. NamedBase Requires one argument
30. model_resource_name = cfp new_helper = new_cfp_url plural_file_name = cfps NamedBase plural_name = cfps plural_route_name = cfps plural_table_name = cfps redirect_resource_name = @cfp show_helper = cfp_url(@cfp)
31. NamedBase Requires one argument
32. singular_name = cfp singular_route_name = cfp singular_table_name = cfp NamedBase table_name = cfps
33. Let's throw all of that away. :)
34. Rails::Generators::Base
35. NamedBase Base Accepts one argument - name No arguments Tons of simple helper methods based on name No helper methods Inherits from base No name
36. Arguments and Options
37. arguments argument :event, type: :string, desc: "event slug" bin/rails g cfp rails-world-2026 required or default dependent on order
38. class_option :event, type: :string, desc: "event-slug" options bin/rails g cfp --event rails-world-2026 Required, default, or optional Order independent
39. Argument Option argument :event, type: :string, desc: "event slug" class_option :event, type: :string, desc: "event-slug" bin/rails g cfp rails-world-2026 bin/rails g cfp --event rails-world-2026 Required or Default Required, default, or optional Order-dependent Order independent
40. class_option
41. bin/rails g cfp --help
42. Help Documentation
43. Help Documentation
44. Option documentation
45. Help Documentation
46. USAGE
47. USAGE files are great for injecting context.
48. USAGE
49. USAGE
50. USAGE
51. USAGE
52. USAGE
53. USAGE
54. Humans struggle with reading documentation too…
55. But I didn't even know this existed.
56. Generating Files
57. data/rails-world/rails-world-2026/cfp.yml
58. lib/generators/cfp/templates/cfp.yml.tt
59. lib/generators/cfp/templates/cfp.yml.tt
60. You can customize templates for the default generators.
61. Rails Guide for Generators
62. cfp_generator.rb
63. source, destination create_cfp_file
64. cfp_generator.rb
65. cfp_generator.rb
66. cfp_generator.rb
67. cfp_generator.rb
68. Let's run it!
69. bin/rails g cfp
70. bin/rails g cfp
71. We have a working generator!!
72. I mentioned the ongoing meetup CFPs?
73. bin/rails g cfp
74. bin/rails g cfp
75. bin/rails g cfp
76. bin/rails g cfp
77. bin/rails g cfp
78. bin/rails g cfp
79. We trimmed whitespace using <%-
80. We need a test.
81. cfp_generator_test
82. cfp_generator_test
83. bundle exec rails test test/lib/generators/cfp_generator_test.rb
84. cfp_generator_test
85. bundle exec rails test test/lib/generators/cfp_generator_test.rb
86. cfp_generator_test
87. bundle exec rails test test/lib/generators/cfp_generator_test.rb
88. Green dots don't inspire much confidence.
89. tmp/generators/data/rails-world/rails-world-2025/cfp.yml
90. Wait. If it persists, why doesn't it conflict?
91. cfp_generator_test
92. cfp_generator_test
93. cfp_generator.rb
94. cfp_generator_test
95. railties/lib/rails/generators/testing/behavior.rb
96. Wouldn't this be flaky if I had parallel test cases?
97. YEAH. IT DOES.
98. cfp_generator_test
99. Use Ruby's temp directories as a destination and cleanup in teardown.
100. But now I can't see the file.
101. Add a binding.irb
102. tmp/cfp_generator20260824-68187-24qeok/data/rails-world/rails-world-2025/cfp.yml
103. bundle exec rails test test/lib/generators/cfp_generator_test.rb
104. Ctrl+C, Quit, Panic!
105. quit + [Enter]?
106. It's like it captured my binding.irb output!!
107. cfp_generator_test
108. RAILS_LOG_TO_STDOUT=true bundle exec rails test test/lib/generators/cfp_generator_test.rb
109. RAILS_LOG_TO_STDOUT=true bin/rails test test/lib/generators
110. Testing Generators Use RAILS_LOG_TO_STDOUT=true to see output Replace destination_root with a tmp directory Don't prepare_destination
111. I have now taught you enough about generators to be dangerous.
112. Resources https://garrettdimon.com/journal/topics/generators https://guides.rubyonrails.org/generators.html https://github.com/rubyevents/rubyevents/tree/main/lib/generators https://github.com/rails/rails/tree/main/railties/lib/rails/generators
113. What about the SKILL?
114. .agents/skills/event-data/SKILL.md
115. .agents/skills/event-data/SKILL.md
116. .agents/skills/event-data/SKILL.md
117. .agents/skills/event-data/SKILL.md
118. MCP SKILL + CLI AI-native SDK that fetches context for a model Markdown documentation discoverable by AI paired with a normal CLI Preferred by AI Usable without tokens Context consumed in describing MCP tool Documentation and Context in Skill Context = Tokens ($) Validations in Skill
119. I said… or MCP, but…
120. What if it were AND MCP?
121. AND MCP
122. The Full Story
123. ToolGeneration on CfpGenerator
124. IT'S A CLI AND MCP!
125. Generator changes are reflected in MCP tools
126. I can test SKILL+CLI vs MCP
127. How'd it go? Method AI - no g AI - g SKILL + CLI MCP Tokens Mistakes
128. GPT 5.4 - No Skill, No Generator, No MCP "Add Rails World 2026, which is described at RAILS_WORLD_2026.md . Use bin/rails validate:all to validate." Old sponsors logos from 2023 1password instead of 1Password Tablecheck vs TableCheck Added a url for the palmer events center 4 Errors
129. How'd it go? Method Tokens Mistakes AI - no g 136,479 ($1.83) 4 Errors AI - g SKILL + CLI MCP
130. GPT 5.4 - No Skill, Generators "Using custom generators, add Rails World 2026, which is described at RAILS_WORLD_2026.md . Use bin/rails validate:all to validate." Forgot PlanetScale Replaced em-dash with hyphens for descriptions 2 Errors
131. How'd it go? Method Tokens Mistakes AI - no g 136,479 ($1.83) 4 Errors AI - g 131,307 ($1.37) 2 Errors SKILL + CLI MCP
132. GPT 5.4 - SKILL, Generators "/event-data add Rails World 2026, which is described at RAILS_WORLD_2026.md . Use bin/rails validate:all to validate." Replace em-dash in descriptions with hyphens 1 Error
133. How'd it go? Method Tokens Mistakes AI - no g 136,479 ($1.83) 4 Errors AI - g 131,307 ($1.37) 2 Errors SKILL + CLI 153,325 ($1.91) 1 Error MCP
134. GPT 5.4 "Using rubyevents mcp, add Rails World 2026, which is described at RAILS_WORLD_2026.md . Use bin/rails validate:all to validate." No videos. No schedule. No sponsors. 3 Major Errors
135. How'd it go? Method Tokens Mistakes AI - no g 136,479 ($1.83) 4 Errors AI - g 131,307 ($1.37) 2 Errors SKILL + CLI 153,325 ($1.91) 1 Error MCP 56,632 ($0.43) 3 Major Errors
136. GPT 5.4-mini
137. How'd it go? Method Tokens Mistakes AI - no g 126,726 ($0.35) 3 Major Errors AI - g 106,753 ($0.41) 2 Errors, 2 Mistakes SKILL + CLI 113,807 ($0.39) 0 Mistakes MCP 65,717 ($0.27) 2 Errors
138. Smaller models with better tools outperformed larger models!
139. rubyonrails.org/ai
140. GPT 5.6 - Luna
141. How'd it go? Method Tokens Mistakes AI - no g 52,951 ($0.04) 2 Errors AI - g 57,785 ($0.04) 2 Errors SKILL + CLI 93,374 ($0.13) 1 Error MCP 43,082 ($0.03) 2 Errors
142. SKILLs had fewer mistakes, but are more expensive.
143. The cost of MCP tooling might be overblown.
144. Build a custom generator! Remind your agents to use Rails generators. Takeaways Use a small model with better tools. Customize existing generator templates! Build for humans too.
145. Humans and AI need help understanding our codebases. Let's build tools for both!
146. Thank you! Chael or Rachael (she/her) @ChaelCodes https://chael.codes/links hello@chael.codes Twitch/YouTube: Sundays at 18:00 UTC