The Elusive Attribute

Talk on the internals of attributes in ActiveModel and ActiveRecord at RailsConf 2019.

Video: https://www.youtube.com/watch?v=PNNrmNTQx2s
Summary: https://railsconf.com/program/sessions#session-751

References:
- Give GeneratedAttributeMethods module a name: https://github.com/rails/rails/pull/35595
- Make plain matcher match first, not last: https://github.com/rails/rails/pull/36005 (PR #36005, mentioned in slide 77)
- Velocipedia: https://www.behance.net/gallery/35437979/Velocipedia

Transcript

1. The Elusive Attribute Chris Salzberg
2. As we know, there are known knowns; there are things we know we know. We also know there are known unknowns; that is to say we know there are some things we do not know. But there are also unknown unknowns - the ones we don't know we don't know. - Donald Rumsfeld United States Secretary of Defense (2003)
9. People who are really into bikes [...] start out from the frame. They represent that small percentage of people who get the drawing perfectly right. Most other people tend to start from what they are most sure of, so for nearly everyone it’s the wheels. [...] I saw many people drawing all the rest in the hope that positioning handlebar, saddle and pedals would help them figure out the frame. - Gianluca Gimini
14. @attributes Name: Chris Salzberg Handle: shioyama Born: Montreal, CA Based: Tokyo, JP Company: Degica Blog: dejimata.com Tags: Module Builder, Mobility
15. 1. methods & columns
17. class Post < ApplicationRecord end < 6.0
18. class Post < ApplicationRecord end Post.ancestors => [Post, Post::GeneratedAssociationMethods, #<AR::AttributeMethods::GeneratedAttributeMethods:...>, ApplicationRecord(abstract), ApplicationRecord::GeneratedAssociationMethods, #<AR::AttributeMethods::GeneratedAttributeMethods:...>, AR::Base, ... ] < 6.0
19. class Post < ApplicationRecord end Post.ancestors => [Post, Post::GeneratedAssociationMethods, #<AR::AttributeMethods::GeneratedAttributeMethods:...>, ApplicationRecord(abstract), ApplicationRecord::GeneratedAssociationMethods, #<AR::AttributeMethods::GeneratedAttributeMethods:...>, AR::Base, ... ] < 6.0
20. class Post < ApplicationRecord end Post.ancestors => [Post, Post::GeneratedAssociationMethods, Post::GeneratedAttributeMethods, ApplicationRecord(abstract), ApplicationRecord::GeneratedAssociationMethods, ApplicationRecord::GeneratedAttributeMethods, ActiveRecord::Base, ... ] >= 6.0
21. post = Post.new(title: "The Elusive Attribute")
22. post = Post.new(title: "The Elusive Attribute") post.method(:title) => #<Method: Post(id: integer, title: string... )
23. post = Post.new(title: "The Elusive Attribute") post.method(:title).owner => Post::GeneratedAttributeMethods
24. => [ ... :title_before_type_cast, :title_came_from_user?, :title?, :title_changed?, :title_change, :title_will_change!, :title_was, :title_previously_changed?, ... ] post = Post.new(title: "The Elusive Attribute") post.method(:title).owner.instance_methods Attribute Methods
27. post = Post.new(title: "The Elusive Attribute") mod = post.method(:title).owner
28. post = Post.new(title: "The Elusive Attribute") mod = post.method(:title).owner mod.instance_methods.each do |meth| mod.send(:remove_method, meth) end
29. post = Post.new(title: "The Elusive Attribute") mod = post.method(:title).owner mod.instance_methods.each do |meth| mod.send(:remove_method, meth) end mod.instance_methods #=> []
30. post = Post.new(title: "The Elusive Attribute") mod = post.method(:title).owner mod.instance_methods.each do |meth| mod.send(:remove_method, meth) end mod.instance_methods post.method(:title) # raises NameError: undefined method `title'
31. post = Post.new(title: "The Elusive Attribute") mod = post.method(:title).owner mod.instance_methods.each do |meth| mod.send(:remove_method, meth) end mod.instance_methods post.method(:title) post.title #=> ...
32. post = Post.new(title: "The Elusive Attribute") mod = post.method(:title).owner mod.instance_methods.each do |meth| mod.send(:remove_method, meth) end mod.instance_methods post.method(:title) post.title #=> "The Elusive Attribute"
35. cache = AR::Base.connection.schema_cache
36. cache = AR::Base.connection.schema_cache columns_hash = cache.instance_variable_get(:@columns_hash) #=> {}
37. cache = AR::Base.connection.schema_cache columns_hash = cache.instance_variable_get(:@columns_hash) columns_hash["posts"] = {}
38. cache = AR::Base.connection.schema_cache columns_hash = cache.instance_variable_get(:@columns_hash) columns_hash["posts"] = {} Post #=> Post()
39. cache = AR::Base.connection.schema_cache columns_hash = cache.instance_variable_get(:@columns_hash) columns_hash["posts"] = {} post = Post.new #=> #<Post:0x00007f4364712df8>
40. cache = AR::Base.connection.schema_cache columns_hash = cache.instance_variable_get(:@columns_hash) columns_hash["posts"] = {} post = Post.new post.method(:title=) # raises NameError: undefined method `title='
41. cache = AR::Base.connection.schema_cache columns_hash = cache.instance_variable_get(:@columns_hash) columns_hash["posts"] = {} post = Post.new post.method(:title=) post.title = "The Elusive Attribute" # raises ActiveModel::UnknownAttributeError
42. cache = AR::Base.connection.schema_cache columns_hash = cache.instance_variable_get(:@columns_hash) columns_hash["posts"] = {} post = Post.new post.method(:title=) post.title = "The Elusive Attribute" post.attributes #=> {}
43. cache = AR::Base.connection.schema_cache columns_hash = cache.instance_variable_get(:@columns_hash) columns_hash["posts"] = {} post = Post.first # SELECT "posts".* FROM "posts" ... #=> #<Post:0x000055c134208af0>
44. cache = AR::Base.connection.schema_cache columns_hash = cache.instance_variable_get(:@columns_hash) columns_hash["posts"] = {} post = Post.first post.method(:title) # raises NameError: undefined method `title'
45. cache = AR::Base.connection.schema_cache columns_hash = cache.instance_variable_get(:@columns_hash) columns_hash["posts"] = {} post = Post.first post.method(:title) post.title #=> ?
46. cache = AR::Base.connection.schema_cache columns_hash = cache.instance_variable_get(:@columns_hash) columns_hash["posts"] = {} post = Post.first post.method(:title) post.title #=> "The Elusive Attribute"
47. cache = AR::Base.connection.schema_cache columns_hash = cache.instance_variable_get(:@columns_hash) columns_hash["posts"] = {} post = Post.first post.method(:title) post.title post.attributes #=> {"id"=>1, "title"=>"The Elusive Attribute", # "created_at"=>2019-03-13 05:09:10 UTC, # "updated_at"=>2019-03-29 07:57:54 UTC}
48. result = AR::Base.connection.exec_query( 'SELECT posts.* FROM posts') #=> #<ActiveRecord::Result:0x...>
49. result = AR::Base.connection.exec_query( 'SELECT posts.* FROM posts') #=> #<ActiveRecord::Result:0x...> result.columns #=> ["id", "title", "created_at", "updated_at"] result.rows #=> [[1, "The Elusive Attribute", "2019-03-15...
50. result = AR::Base.connection.exec_query( 'SELECT posts.* FROM posts') #=> #<ActiveRecord::Result:0x...> result.columns #=> ["id", "title", "created_at", "updated_at"] result.rows #=> [[1, "The Elusive Attribute", "2019-03-15... Post.column_names #=> []
55. Post.select('title as foo')
56. post = Post.select('title as foo').first post.foo #=> "The Elusive Attribute"
58. 2. Matchers & Dispatchers
59. Constraints
60. Constraints 1. Methods are fast, method missing is slow
61. Constraints 1. Methods are fast, method missing is slow 2. schema is static, attributes are dynamic
62. Constraints 1. Methods are fast, method missing is slow 2. schema is static, attributes are dynamic “front door” “back door”
63. prefix: "" suffix: "_was" Matcher (Attribute Method)
64. prefix: "" suffix: "_was" "title" attr_name
65. prefix: "" suffix: "_was" "title_was" "title" method_name
66. prefix: "" suffix: "_was" "title_was" "attribute_was" "title" target
67. prefix: "" suffix: "_was" def title_was(*args) attribute_was("title", *args) end "title" Dispatcher
68. “Front Door” attribute_method_matchers.each do |matcher| method_name = matcher.method_name(attr_name) define_proxy_call true, generated_attribute_methods, method_name, matcher.target, attr_name.to_s end
69. “Front Door” attribute_method_matchers.each do |matcher| method_name = matcher.method_name(attr_name) define_proxy_call true, generated_attribute_methods, method_name, matcher.target, attr_name.to_s end “title”
70. “Front Door” attribute_method_matchers.each do |matcher| method_name = matcher.method_name(attr_name) define_proxy_call true, generated_attribute_methods, method_name, matcher.target, attr_name.to_s end “title_was” “attribute_was” “title”
71. prefix: "" suffix: "_was"
72. regex: /^(.*)_was$/
73. "foo_was" regex: /^(.*)_was$/ method_name
74. "foo" regex: /^(.*)_was$/ match("foo_was") attr_name
75. "foo", "attribute_was" regex: /^(.*)_was$/ match("foo_was") target
76. “Back Door” def attribute_method_matchers_matching(method_name) matchers = attribute_method_matchers. partition(&:plain?).reverse.flatten(1) matchers.map { |matcher| matcher.match(method_name) }.compact end
77. “Back Door” def attribute_method_matchers_matching(method_name) matchers = attribute_method_matchers. partition(&:plain?).reverse.flatten(1) matchers.map { |matcher| matcher.match(method_name) }.compact end (See PR #36005)
78. “Back Door” def attribute_method_matchers_matching(method_name) matchers = attribute_method_matchers. partition(&:plain?).reverse.flatten(1) matchers.map { |matcher| matcher.match(method_name) }.compact end “foo_was”
87. module ActiveModel::Dirty extend ActiveSupport::Concern include ActiveModel::AttributeMethods included do attribute_method_suffix "_changed?", "_change", "_will_change!", "_was" # Dispatch target for *_was attribute methods. def attribute_was(attr) attribute_changed?(attr) ? changed_attributes[attr] : _read_attribute(attr) end
88. module ActiveModel::Dirty extend ActiveSupport::Concern include ActiveModel::AttributeMethods included do attribute_method_suffix "_changed?", "_change", "_will_change!", "_was" # Dispatch target for *_was attribute methods. def attribute_was(attr) attribute_changed?(attr) ? changed_attributes[attr] : _read_attribute(attr) end
89. module ActiveModel::AttributeMethods included do class_attribute :attribute_method_matchers, instance_writer: false, default: [ ClassMethods::AttributeMethodMatcher.new ] # ... end end
90. module ActiveModel::AttributeMethods included do class_attribute :attribute_method_matchers, instance_writer: false, default: [ ClassMethods::AttributeMethodMatcher.new ] # ... end end
91. module ActiveModel::AttributeMethods included do class_attribute :attribute_method_matchers, instance_writer: false, default: [ ClassMethods::AttributeMethodMatcher.new ] # ... end end def initialize(options = {}) @prefix = options.fetch(:prefix, "") @suffix = options.fetch(:suffix, "")
94. module ActiveRecord module AttributeMethods module Read # ... def _read_attribute(attr_name, &b) @attributes.fetch_value(attr_name.to_s, &b) end alias :attribute :_read_attribute private :attribute end end end
97. 3. A Better Frame
99. post = Post.select('title as foo').first post.foo #=> "The Elusive Attribute"
100. post = Post.select('title as foo').first post.foo #=> "The Elusive Attribute" post.foo = "This cannot be saved"
101. post = Post.select('title as foo').first post.foo #=> "The Elusive Attribute" post.foo = "This cannot be saved" post.foo_was
102. post = Post.select('title as foo').first post.foo #=> "The Elusive Attribute" post.foo = "This cannot be saved" post.foo_was post.foo_change
103. post = Post.select('1 as one').first post.one #=> 1 post.one = 2 post.one_was post.one_change
104. post = Post.select('1 as one').first post.one #=> 1 post.one = 2 # post.one #=> 2 post.one_was #=> 1 post.one_change #=> [1, 2]
105. payments = Payment.select( 'amount, amount*0.08 as tax, amount*0.02 as tax_change')
106. payments = Payment.select( 'amount, amount*0.08 as tax, amount*0.02 as tax_change') payment = payments.first payment.amount #=> 100 payment.tax #=> 8 payment.tax_change #=> ?
107. payments = Payment.select( 'amount, amount*0.08 as tax, amount*0.02 as tax_change') payment = payments.first payment.amount #=> 100 payment.tax #=> 8 payment.tax_change #=> nil
108. "tax", "attribute_change" regex: /^(.*)_change$/ match("tax_change")
110. DON’T JUST UNPACK IT,
111. DON’T JUST UNPACK IT, FIX IT
112. Chris Salzberg @shioyama /dejimata.com