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
   

   View Slide

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)
   

   View Slide

3. View Slide

4. View Slide

5. View Slide

6. View Slide

7. View Slide

8. View Slide

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
   

   View Slide

10. View Slide

11. View Slide

12. View Slide

13. View Slide

14. @attributes
    Name: Chris Salzberg
    Handle: shioyama
    Born: Montreal, CA
    Based: Tokyo, JP
    Company: Degica
    Blog: dejimata.com
    Tags: Module Builder, Mobility
    

    View Slide

15. 1. methods & columns
    

    View Slide

16. View Slide

17. class Post < ApplicationRecord
    end
    < 6.0
    

    View Slide

18. class Post < ApplicationRecord
    end
    Post.ancestors
    => [Post,
    Post::GeneratedAssociationMethods,
    #,
    ApplicationRecord(abstract),
    ApplicationRecord::GeneratedAssociationMethods,
    #,
    AR::Base,
    ... ]
    < 6.0
    

    View Slide

19. class Post < ApplicationRecord
    end
    Post.ancestors
    => [Post,
    Post::GeneratedAssociationMethods,
    #,
    ApplicationRecord(abstract),
    ApplicationRecord::GeneratedAssociationMethods,
    #,
    AR::Base,
    ... ]
    < 6.0
    

    View Slide

20. class Post < ApplicationRecord
    end
    Post.ancestors
    => [Post,
    Post::GeneratedAssociationMethods,
    Post::GeneratedAttributeMethods,
    ApplicationRecord(abstract),
    ApplicationRecord::GeneratedAssociationMethods,
    ApplicationRecord::GeneratedAttributeMethods,
    ActiveRecord::Base,
    ... ]
    >= 6.0
    

    View Slide

21. post = Post.new(title: "The Elusive Attribute")
    

    View Slide

22. post = Post.new(title: "The Elusive Attribute")
    post.method(:title)
    => #)
    

    View Slide

23. post = Post.new(title: "The Elusive Attribute")
    post.method(:title).owner
    => Post::GeneratedAttributeMethods
    

    View Slide

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
    

    View Slide

25. View Slide

26. View Slide

27. post = Post.new(title: "The Elusive Attribute")
    mod = post.method(:title).owner
    

    View Slide

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
    

    View Slide

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 #=> []
    

    View Slide

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'
    

    View Slide

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
    #=> ...
    

    View Slide

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"
    

    View Slide

33. View Slide

34. View Slide

35. cache = AR::Base.connection.schema_cache
    

    View Slide

36. cache = AR::Base.connection.schema_cache
    columns_hash =
    cache.instance_variable_get(:@columns_hash)
    #=> {}
    

    View Slide

37. cache = AR::Base.connection.schema_cache
    columns_hash =
    cache.instance_variable_get(:@columns_hash)
    columns_hash["posts"] = {}
    

    View Slide

38. cache = AR::Base.connection.schema_cache
    columns_hash =
    cache.instance_variable_get(:@columns_hash)
    columns_hash["posts"] = {}
    Post
    #=> Post()
    

    View Slide

39. cache = AR::Base.connection.schema_cache
    columns_hash =
    cache.instance_variable_get(:@columns_hash)
    columns_hash["posts"] = {}
    post = Post.new
    #=> #
    

    View Slide

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='
    

    View Slide

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
    

    View Slide

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
    #=> {}
    

    View Slide

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" ...
    #=> #
    

    View Slide

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'
    

    View Slide

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
    #=> ?
    

    View Slide

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"
    

    View Slide

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}
    

    View Slide

48. result = AR::Base.connection.exec_query(
    'SELECT posts.* FROM posts')
    #=> #
    

    View Slide

49. result = AR::Base.connection.exec_query(
    'SELECT posts.* FROM posts')
    #=> #
    result.columns
    #=> ["id", "title", "created_at", "updated_at"]
    result.rows
    #=> [[1, "The Elusive Attribute", "2019-03-15...
    

    View Slide

50. result = AR::Base.connection.exec_query(
    'SELECT posts.* FROM posts')
    #=> #
    result.columns
    #=> ["id", "title", "created_at", "updated_at"]
    result.rows
    #=> [[1, "The Elusive Attribute", "2019-03-15...
    Post.column_names
    #=> []
    

    View Slide

51. View Slide

52. View Slide

53. View Slide

54. View Slide

55. Post.select('title as foo')
    

    View Slide

56. post = Post.select('title as foo').first
    post.foo
    #=> "The Elusive Attribute"
    

    View Slide

57. View Slide

58. 2. Matchers & Dispatchers
    

    View Slide

59. Constraints
    

    View Slide

60. Constraints
    1. Methods are fast, method missing is slow
    

    View Slide

61. Constraints
    1. Methods are fast, method missing is slow
    2. schema is static, attributes are dynamic
    

    View Slide

62. Constraints
    1. Methods are fast, method missing is slow
    2. schema is static, attributes are dynamic
    “front door”
    “back door”
    

    View Slide

63. prefix: ""
    suffix: "_was"
    Matcher
    (Attribute Method)
    

    View Slide

64. prefix: ""
    suffix: "_was"
    "title"
    attr_name
    

    View Slide

65. prefix: ""
    suffix: "_was"
    "title_was"
    "title"
    method_name
    

    View Slide

66. prefix: ""
    suffix: "_was"
    "title_was"
    "attribute_was"
    "title"
    target
    

    View Slide

67. prefix: ""
    suffix: "_was"
    def title_was(*args)
    attribute_was("title", *args)
    end
    "title"
    Dispatcher
    

    View Slide

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
    

    View Slide

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”
    

    View Slide

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”
    

    View Slide

71. prefix: ""
    suffix: "_was"
    

    View Slide

72. regex:
    /^(.*)_was$/
    

    View Slide

73. "foo_was"
    regex:
    /^(.*)_was$/
    method_name
    

    View Slide

74. "foo"
    regex:
    /^(.*)_was$/
    match("foo_was")
    attr_name
    

    View Slide

75. "foo", "attribute_was"
    regex:
    /^(.*)_was$/
    match("foo_was")
    target
    

    View Slide

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
    

    View Slide

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)
    

    View Slide

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”
    

    View Slide

79. View Slide

80. View Slide

81. View Slide

82. View Slide

83. View Slide

84. View Slide

85. View Slide

86. View Slide

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
    

    View Slide

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
    

    View Slide

89. module ActiveModel::AttributeMethods
    included do
    class_attribute :attribute_method_matchers,
    instance_writer: false,
    default: [
    ClassMethods::AttributeMethodMatcher.new
    ]
    # ...
    end
    end
    

    View Slide

90. module ActiveModel::AttributeMethods
    included do
    class_attribute :attribute_method_matchers,
    instance_writer: false,
    default: [
    ClassMethods::AttributeMethodMatcher.new
    ]
    # ...
    end
    end
    

    View Slide

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, "")
    

    View Slide

92. View Slide

93. View Slide

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
    

    View Slide

95. View Slide

96. View Slide

97. 3. A Better Frame
    

    View Slide

98. View Slide

99. post = Post.select('title as foo').first
    post.foo
    #=> "The Elusive Attribute"
    

    View Slide

100. post = Post.select('title as foo').first
     post.foo
     #=> "The Elusive Attribute"
     post.foo = "This cannot be saved"
     

     View Slide

101. post = Post.select('title as foo').first
     post.foo
     #=> "The Elusive Attribute"
     post.foo = "This cannot be saved"
     post.foo_was
     

     View Slide

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
     

     View Slide

103. post = Post.select('1 as one').first
     post.one
     #=> 1
     post.one = 2
     post.one_was
     post.one_change
     

     View Slide

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]
     

     View Slide

105. payments = Payment.select(
     'amount,
     amount*0.08 as tax,
     amount*0.02 as tax_change')
     

     View Slide

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 #=> ?
     

     View Slide

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
     

     View Slide

108. "tax", "attribute_change"
     regex:
     /^(.*)_change$/
     match("tax_change")
     

     View Slide

109. View Slide

110. DON’T JUST
     UNPACK IT,
     

     View Slide

111. DON’T JUST
     UNPACK IT,
     FIX IT
     

     View Slide

112. Chris Salzberg
     @shioyama /dejimata.com
     

     View Slide