Anatomy of constant lookup and autoloading in Rails

Transcript

1. Anatomy of constant lookup & autoloading in Rails @_cha1tanya @BigBinary
2. class PostsController < ApplicationController def index @posts = Post.all end end
3. class PostsController < ApplicationController def index @posts = Post.all end end
4. It just works! ™
5. Constants in Ruby
6. class User < ActiveRecord::Base BASE_URL = “http://example.com” end
7. class User < ActiveRecord::Base end
8. User = Class.new(ActiveRecord::Base)
9. module Admin class Users < ActiveRecord::Base end end
10. All class & module definitions create constants
11. class PostsController < ApplicationController def index @posts = Post.all end end
12. We refer to constants while using classes or modules
13. Admin::ApiService::BASE_URL
14. Constants are stored in modules
15. API for handling constants
16. Module.const_set Module.const_get Module.const_missing Module.remove_const
17. Constant lookup in Ruby
18. class ApiController BASE_URL = “https://github.com/api/v1” def print_location puts BASE_URL #=> ? end end
19. Look for the constant inside current scope
20. class ApiController BASE_URL = “https://github.com/api/v1” def print_location puts BASE_URL” end end #=> “https://github.com/api/v1
21. module Admin BASE_URL = “https://github.com/api/v1” class ApiController def print_location puts BASE_URL #=> ? end end end
22. Look for the constant in the parent namespace
23. module Admin BASE_URL = “https://github.com/api/v1” class ApiController def print_location puts BASE_URL end end end #=> “https://github.com/api/v1”
24. module SuperAdmin BASE_URL = “https://github.com/api/v1” module Admin class ApiController < BaseController def print_location puts BASE_URL #=> ? end end end end
25. Keep looking in the parent namespaces
26. module SuperAdmin BASE_URL = “https://github.com/api/v1” module Admin class ApiController < BaseController def print_location puts BASE_URL end end end end #=> “https://github.com/api/v1”
27. class BaseController BASE_URL = “https://github.com/api/v1” end class ApiController < BaseController def print_location puts BASE_URL #=> ? end end
28. class BaseController BASE_URL = “https://github.com/api/v1” end class ApiController < BaseController def print_location puts BASE_URL end end #=> “https://github.com/api/v1”
29. Look for the constant in the ancestors chain
30. class BaseController BASE_URL = “https://github.com/api/v2” end module Admin BASE_URL = “https://github.com/api/v1” class ApiController < BaseController def print_location puts BASE_URL #=> ? end end end
31. module Admin BASE_URL = “https://github.com/api/v1” class ApiController < BaseController def print_location puts BASE_URL end end end #=> “https://github.com/api/v1”
32. class BaseController BASE_URL = “https://github.com/api/v2” end module Admin class ApiController < BaseController def print_location puts BASE_URL end end end #=> “https://github.com/api/v2”
33. Search in namespaces
34. Search in namespaces Search in ancestors tree
35. module SuperAdmin BASE_URL = “https://github.com/api/v1” module Admin BASE_URL = “https://github.com/api/v2” end end
36. module SuperAdmin class Admin::ApiController def print_location puts BASE_URL #=> ? end end end
37. module SuperAdmin class Admin::ApiController def print_location puts BASE_URL #=> ? end end end
38. module SuperAdmin class Admin::ApiController def print_location puts BASE_URL end end end #=> “https://github.com/api/v1”
39. module SuperAdmin BASE_URL = “https://github.com/api/v1” module Admin BASE_URL = “https://github.com/api/v2” end end
40. Module.nesting
41. module SuperAdmin class Admin::ApiController puts Module.nesting end end
42. [SuperAdmin::Admin::ApiController, SuperAdmin]
43. [SuperAdmin::Admin::ApiController, SuperAdmin]
44. SuperAdmin::Admin::ApiController Not Found
45. [SuperAdmin::Admin::ApiController, SuperAdmin]
46. module SuperAdmin BASE_URL = “https://github.com/api/v1” module Admin BASE_URL = “https://github.com/api/v2” end end
47. SuperAdmin::Admin
48. SuperAdmin::Admin Not Checked!
49. Ruby’s constant lookup algorithm**
50. Search in current namespace
51. Continue searching up in parent namespaces**
52. All namespaces are exhausted
53. Search in ancestors chain of the innermost class/module
54. Ancestors chain is exhausted
55. Module.const_missing(const_name)
56. Entry point for Rails!
57. # activesupport/lib/../dependencies.rb def const_missing(const_name) …… end
58. Autoloading in Rails
59. Loading mechanism
60. require v/s load
61. require require’s only once!
62. load will load and execute every time!
63. require in production
64. require in production load in development
65. config.cache_classes
66. Autoload Paths
67. config.autoload_paths
68. app/assets app/channels app/controllers app/controllers/concerns app/helpers app/jobs app/mailers app/models app/models/concerns test/mailers/previews
69. Similar to $LOAD_PATH in Ruby
70. Constant lookup by Rails
71. class PostsController < ApplicationController def index @posts = Post.all end end
72. PostsController ApplicationController Post
73. PostsController
74. class PostsController < ApplicationController def index @posts = Post.all end end
75. Constants after class or module
76. No autoloading by Rails
77. ApplicationController
78. class PostsController < ApplicationController def index @posts = Post.all end end
79. Top level constant
80. app/assets/application_controller.rb
81. app/assets/application_controller.rb app/channels/application_controller.rb
82. app/assets/application_controller.rb app/channels/application_controller.rb app/controllers/application_controller.rb # FOUND!
83. Post
84. class PostsController < ApplicationController def index @posts = Post.all end end
85. PostsController::Post
86. Namespaced constants
87. app/assets/posts_controller/post.rb app/channels/posts_controller/post.rb app/controllers/posts_controller/post.rb …
88. app/assets/posts_controller/post app/channels/posts_controller/post app/controllers/posts_controller/post …
89. Automatic modules
90. module Admin end module Admin class User < ActiveRecord::Base end Admin::User
91. Module acting as namespace
92. app/models/admin
93. app/models/admin app/models/admin/user.rb
94. Admin = Module.new
95. Coming back to Post
96. app/assets/posts_controller/post app/channels/posts_controller/post app/controllers/posts_controller/post …
97. PostsController::Post # Not Found
98. Search for Post constant
99. app/assets/post.rb app/channels/post.rb app/controllers/post.rb … app/models/post.rb # FOUND!
100. Qualified constants
101. module Admin class User < ActiveRecord::Base end end Admin::User # Qualified constant
102. class User < ActiveRecord::Base end User # Top level constant
103. Admin::User
104. admin/user.rb
105. admin/user.rb
106. user.rb
107. user.rb
108. Admin::User
109. Name error when not found
110. Constant lookup by Rails
111. Look for regular .rb file
112. Look for automatic module
113. Repeat search in parent namespace**
114. The file is found, the constant is found
115. File found, constant not defined
116. LoadError
117. Constant not found, File not found
118. NameError
119. Constant Reloading
120. config.cache_classes = false
121. List of monitored files
122. config/routes.rb Locales Ruby files under autoload paths db/schema.rb & db/structure.sql
123. Module.remove_const
124. Wipe out all autoloader constants at the start of request
125. @posts = Post.all
126. Module.const_missing again!
127. Reloading != Reloading
128. Reloading == Wiping
129. Convenient to wipe out & wait for autoload to kick!
130. What happens with Gems?
131. # config/application.rb Bundler.require(*Rails.groups)
132. Gems are already require’d
133. Unless require: false
134. Gotchas
135. Side effects of assumptions by Rails
136. Nesting information is not passed
137. module Admin class User < ActiveRecord::Base end end
138. class User < ActiveRecord::Base end
139. class Admin::UsersController def index @users = User.all end end
140. Admin::User or User?
141. class Admin::UsersController def index @users = User.all end end # app/models/user.rb # In Ruby
142. class Admin::UsersController Module.nesting #=> [Admin::UsersController] end # app/models/user.rb # In Ruby
143. class Admin::UsersController def index @users = User.all end end # app/models/admin/user.rb # In Rails
144. module Admin class UsersController def index @users = User.all end end
145. Prefer relative constants
146. class Admin::UsersController def index @users = Admin::User.all end end # app/models/admin/user.rb
147. Don’t mix require and autoloading
148. require ‘user’ class Admin::UsersController def index @users = User.all end end
149. Convention over configuration
150. Follow the contract and you will be happy!
151. Thanks! @_cha1tanya @BigBinary