RailsConf 2022 - Reflecting on Active Record Associations

Active Record associations seem magical—add a has_many here, a belongs_to there, and suddenly your models are loaded with behavior. Could it be magic, or is it plain old Ruby with some thoughtful design and a bit of metaprogramming? In this talk we'll study Active Record associations by writing our own belongs_to and has_many macros. We'll dynamically define methods, cache query results, replace a a Relation with a CollectionProxy, and automatically prevent N+1 queries with inverses. You'll leave with a deeper understanding of associations, and a new appreciation for their magic.

Transcript

1. Reflecting on Active Record Associations @composerinteralia Daniel Colson @dodecadaniel
2. class Repository < ApplicationRecord has_many :pull_requests end class PullRequest < ApplicationRecord belongs_to :repository end
3. • repository • repository=(repository) • build_repository • create_repository • create_repository! • reload_repository • repository_changed? • repository_previously_changed?
4. • repository • repository=(repository) • build_repository • create_repository • create_repository! • reload_repository • repository_changed? • repository_previously_changed? • Presence Validations • Caching • Autosaving • Preloading • Destroy Callbacks • Scopes • Extensions • Etc.
5. Rails Magic !
6. !!!!!!! "#$%&'(
7. Let’s Study Magic!
8. @composerinteralia Daniel Colson @dodecadaniel
10. Study Guide • Metaprogramming • Reflections • Caching • Relations • Inverses
11. Metaprogramming )
12. Metaprogramming class PullRequest < ApplicationRecord belongs_to :repository end
13. Metaprogramming class PullRequest < ApplicationRecord self.belongs_to(:repository) end Calling a class method
14. Metaprogramming class PullRequest < ApplicationRecord def self.belongs_to(name) end end Defining a class method
15. Metaprogramming Reader Method
16. Metaprogramming > pull_request => <#PullRequest @repository_id=42> Reader Method
17. Metaprogramming > pull_request => <#PullRequest @repository_id=42> > pull_request.repository => <#Repository @id=42> Reader Method
18. Metaprogramming > pull_request => <#PullRequest @repository_id=42> > pull_request.repository => <#Repository @id=42> Primary Key Foreign Key Reader Method
19. Metaprogramming def self.belongs_to(name) def repository Repository.where(id: repository_id).first end end Reader Method
20. Metaprogramming Writer Method
21. Metaprogramming > pull_request => <#PullRequest @repository_id=42> Writer Method
22. Metaprogramming > pull_request => <#PullRequest @repository_id=42> > repository => <#Repository @id=77> Writer Method
23. Metaprogramming > pull_request => <#PullRequest @repository_id=42> > repository => <#Repository @id=77> > pull_request.repository = repository => <#PullRequest @repository_id=77> Writer Method
24. Metaprogramming def self.belongs_to(name) def repository Repository.where(id: repository_id).first end def repository=(repository) self.repository_id = repository.id end end Writer Method
25. Metaprogramming def self.belongs_to(name) def repository Repository.where(id: repository_id).first end def repository=(record) self.repository_id = repository.id end end Hardcoded Hardcoded
26. Metaprogramming def self.belongs_to(name) define_method(name) do Repository.where(id: repository_id).first end define_method("#{name}=") do |record| self.repository_id = repository.id end end Generic Generic
27. Metaprogramming def self.belongs_to(name) define_method(name) do Repository.where(:id => self[:repository_id]).first end define_method("#{name}=") do |record| self[:repository_id] = record[:id] end end Hardcoded
28. def self.belongs_to(name) define_method(name) do klass.where(primary_key => self[foreign_key]).first end define_method("#{name}=") do |record| self[foreign_key] = record[primary_key] end end Metaprogramming Generic
29. Metaprogramming class PullRequest < ApplicationRecord belongs_to :repository end
30. Metaprogramming class PullRequest < ApplicationRecord belongs_to :repository end klass => Repository primary_key => :id foreign_key => :repository_id ?
31. Reflections *
32. Reflections class Reflection def initialize(name, active_record) @active_record = active_record @name = name end end
33. Reflections Reflection @active_record = PullRequest @name = :repository
34. Reflections • #klass • #primary_key • #foreign_key Reflection @active_record = PullRequest @name = :repository
35. Reflection @active_record = PullRequest @name = :repository Reflections • #klass => Repository • #primary_key • #foreign_key
36. Reflection @active_record = PullRequest @name = :repository Reflections • #klass => Repository • #primary_key • #foreign_key
37. Reflections class Reflection def klass @name.to_s.camelize.constantize end end
38. Reflections • #klass => Repository • #primary_key => :id • #foreign_key Reflection @active_record = PullRequest @name = :repository
39. Reflections class Reflection def primary_key klass.primary_key end end
40. Reflection @active_record = PullRequest @name = :repository Reflections • #klass => Repository • #primary_key => :id • #foreign_key => :repository_id
41. Reflection @active_record = PullRequest @name = :repository Reflections • #klass => Repository • #primary_key => :id • #foreign_key => :repository_id
42. Reflections class Reflection def foreign_key "#{@name}_id" end end
43. Reflections def self.belongs_to(name) reflection = Reflection.new(name, self)
44. Reflections def self.belongs_to(name) reflection = Reflection.new(name, self) klass = reflection.klass primary_key = reflection.primary_key foreign_key = reflection.foreign_key
45. def self.belongs_to(name) define_method(name) do klass.where(primary_key => self[foreign_key]).first end define_method("#{name}=") do |record| self[foreign_key] = record[primary_key] end end Reflections
46. Caching +
47. Caching > repo = pull_request.repository => #<Repository:0x307468 @id=42> > same_repo = pull_request.repository => #<Repository:0x647570 @id=42>
48. Caching > repo = pull_request.repository => #<Repository:0x307468 @id=42> > same_repo = pull_request.repository => #<Repository:0x647570 @id=42> Different objects
49. Caching > repo = pull_request.repository => #<Repository:0x307468 @id=42> > same_repo = pull_request.repository => #<Repository:0x647570 @id=42> > repo.name = "new_name" > same_repo.name => "old_name" Different objects
50. Caching > repo = pull_request.repository => #<Repository:0x307468 @id=42> > same_repo = pull_request.repository => #<Repository:0x647570 @id=42> > repo.name = "new_name" > same_repo.name => "old_name" Different objects Inconsistent data ,
51. Caching class Association def initialize(owner, reflection) @owner = owner @reflection = reflection @loaded = false @target = nil end end
52. Caching def self.belongs_to(name) define_method(name) do klass.where(primary_key => self[foreign_key]).first end define_method("#{name}=") do |record| self[foreign_key] = record[primary_key] end end
53. Caching def self.belongs_to(name) define_method(name) do end define_method("#{name}=") do |record| end end
54. Caching class Association def reader end def writer(record) end end Association Reader Association Writer
55. Caching class Association def reader klass.where(primary_key => @owner[foreign_key]).first end def writer(record) @owner[foreign_key] = record[primary_key] end end Association Reader Association Writer
56. Caching def self.belongs_to(name) define_method(name) do association(name).reader end define_method("#{name}=") do |record| association(name).writer(record) end end Use Association Reader and Writer
57. Caching def self.belongs_to(name) define_method(name) do association(name).reader end define_method("#{name}=") do |record| association(name).writer(record) end end Use Association Reader and Writer Get Association by name
58. Caching Association @owner = #<PullRequest> @reflection = #<Reflection> @loaded = false @target = nil
59. Caching def reader if loaded? else end end Association @owner = #<PullRequest> @reflection = #<Reflection> @loaded = false @target = nil
60. Caching def reader if loaded? else end end Association @owner = #<PullRequest> @reflection = #<Reflection> @loaded = false @target = nil
61. Association @owner = #<PullRequest> @reflection = #<Reflection> @loaded = false @target = nil Caching def reader if loaded? else self.target = klass.where(...).first end end
62. Association @owner = #<PullRequest> @reflection = #<Reflection> @loaded = false @target = nil Caching def reader if loaded? else self.target = klass.where(...).first end end true #<Repository>
63. Caching def reader if loaded? else self.target = klass.where(...).first end end Association @owner = #<PullRequest> @reflection = #<Reflection> @loaded = true @target = #<Repository>
64. Caching def reader if loaded? target else self.target = klass.where(...).first end end Association @owner = #<PullRequest> @reflection = #<Reflection> @loaded = true @target = #<Repository> Association @owner = #<PullRequest> @reflection = #<Reflection> @loaded = true @target = #<Repository>
65. Caching def writer(record) @owner[foreign_key] = record[primary_key] self.target = record end Update target Beware of a stale cache!
66. Caching > repo = pull_request.repository => #<Repository:0x776f6e @id=42> > same_repo = pull_request.repository => #<Repository:0x776f6e @id=42>
67. Caching > repo = pull_request.repository => #<Repository:0x776f6e @id=42> > same_repo = pull_request.repository => #<Repository:0x776f6e @id=42> Same object
68. Caching > repo = pull_request.repository => #<Repository:0x776f6e @id=42> > same_repo = pull_request.repository => #<Repository:0x776f6e @id=42> > repo.name = "new_name" > same_repo.name => "new_name" Same object
69. Caching > repo = pull_request.repository => #<Repository:0x776f6e @id=42> > same_repo = pull_request.repository => #<Repository:0x776f6e @id=42> > repo.name = "new_name" > same_repo.name => "new_name" Same object Consistent data -
70. Relations .
71. Relations class Repository < ApplicationRecord has_many :pull_requests end
72. def self.has_many(name) define_method(name) do association(name).reader end define_method("#{name}=") do |record| association(name).writer(record) end end Relations
73. Relations class BelongsToAssociation < Association end class HasManyAssociation < Association end
74. def reader if loaded? target else self.target = klass.where(primary_key => @owner[foreign_key]).first end end Relations BelongsToAssociation#reader
75. def reader if loaded? target else self.target = klass.where(primary_key => @owner[foreign_key]).first end end Relations BelongsToAssociation#reader Primary Key Foreign Key
76. def reader if loaded? target else self.target = klass.where(primary_key => @owner[foreign_key]).first end end Relations BelongsToAssociation#reader Primary Key Foreign Key Single Record
77. def reader if loaded? target else self.target = klass.where(foreign_key => @owner[primary_key]).to_a end end Relations HasManyAssociation#reader
78. def reader if loaded? target else self.target = klass.where(foreign_key => @owner[primary_key]).to_a end end Relations Foreign Key Primary Key HasManyAssociation#reader
79. def reader if loaded? target else self.target = klass.where(foreign_key => @owner[primary_key]).to_a end end Relations Foreign Key Primary Key Array of Records HasManyAssociation#reader
80. Relations > repository.pull_requests => SELECT * FROM pull_requests WHERE repository_id = 42 => [#<PullRequest @repository_id=42>]
81. Relations > repository.pull_requests => SELECT * FROM pull_requests WHERE repository_id = 42 => [#<PullRequest @repository_id=42>] > repository.pull_requests => [#<PullRequest @repository_id=42>]
82. Relations What is a Relation?
83. Relations > pull_requests = PullRequest.where(repository_id: 42) => <#ActiveRecord::Relation> What is a Relation?
84. Relations > pull_requests = PullRequest.where(repository_id: 42) => <#ActiveRecord::Relation> • Super-powered array of records • Lazy loading • #create • And much more What is a Relation?
85. Relations > pull_requests = repository.pull_requests => <#ActiveRecord::Relation> Association Relations?
86. Relations > pull_requests = repository.pull_requests => <#ActiveRecord::Relation> Association @loaded = false @target = nil Association Relations?
87. Relations > pull_requests = repository.pull_requests => <#ActiveRecord::Relation> > pull_requests.to_a Association @loaded = false @target = nil Association Relations?
88. Relations > pull_requests = repository.pull_requests => <#ActiveRecord::Relation> > pull_requests.to_a Association @loaded = false @target = nil true [<#PullRequest>] => [<#PullRequest>] Association Relations?
89. Relations class CollectionProxy < ActiveRecord::Relation def initialize(association) @association = association super end end
90. def reader CollectionProxy.new(self) end Relations Updated Reader HasManyAssociation#reader
91. def load_target if loaded? target else self.target = klass.where(foreign_key => @owner[primary_key]).to_a end end Relations HasManyAssociation#load_target Moved From Reader
92. HasManyAssociation @loaded = false @target = nil repository.pull_requests CollectionProxy @association = Relations
93. HasManyAssociation @loaded = false @target = nil repository.pull_requests repository.pull_requests.to_a CollectionProxy @association = Relations
94. HasManyAssociation @loaded = false @target = nil repository.pull_requests repository.pull_requests.to_a CollectionProxy @association = true [] => [] Relations
95. repository.pull_requests repository.pull_requests.to_a => [] repository.pull_requests.create HasManyAssociation @loaded = true @target = [] Relations CollectionProxy @association =
96. repository.pull_requests repository.pull_requests.to_a => [] repository.pull_requests.create HasManyAssociation @loaded = true @target = [] Relations CollectionProxy @association = [#<PullRequest>] => #<PullRequest>
97. repository.pull_requests repository.pull_requests.to_a => [] repository.pull_requests.create repository.pull_requests.to_a => [#<PullRequest>] HasManyAssociation @loaded = true @target = [] Relations CollectionProxy @association = [#<PullRequest>] => #<PullRequest>
98. Inverses /
99. Inverses class Repository < ApplicationRecord has_many :pull_requests end class PullRequest < ApplicationRecord belongs_to :repository end
100. Inverses > pull_requests = repository.pull_requests => SELECT * FROM pull_requests WHERE repository_id = 42
101. Inverses > pull_requests = repository.pull_requests => SELECT * FROM pull_requests WHERE repository_id = 42 > pull_requests.map(&:repository)
102. Inverses > pull_requests = repository.pull_requests => SELECT * FROM pull_requests WHERE repository_id = 42 > pull_requests.map(&:repository) => SELECT * FROM repositories WHERE id = 42 LIMIT 1 => SELECT * FROM repositories WHERE id = 42 LIMIT 1 => SELECT * FROM repositories WHERE id = 42 LIMIT 1 => SELECT * FROM repositories WHERE id = 42 LIMIT 1 => SELECT * FROM repositories WHERE id = 42 LIMIT 1 0
103. BelongsToAssociation @owner = #<PullRequest> @loaded = false @target = nil Inverses
104. BelongsToAssociation @owner = #<PullRequest> @loaded = false @target = nil pull_request.repository Inverses
105. BelongsToAssociation @owner = #<PullRequest> @loaded = false @target = nil pull_request.repository Inverses HasManyAssociation @owner = #<Repository> @loaded = true @target = [#<PullRequest>, ...] repository.pull_requests
106. BelongsToAssociation @owner = #<PullRequest> @loaded = false @target = nil pull_request.repository Inverses true #<Repository> HasManyAssociation @owner = #<Repository> @loaded = true @target = [#<PullRequest>, ...] repository.pull_requests
107. Inverses HasManyAssociation @owner = #<Repository> @loaded = true @target = [#<PullRequest>, ...]
108. Inverses @target.each do |record| HasManyAssociation @owner = #<Repository> @loaded = true @target = [#<PullRequest>, ...]
109. Inverses @target.each do |record| #<PullRequest> HasManyAssociation @owner = #<Repository> @loaded = true @target = [#<PullRequest>, ...]
110. Inverses @target.each do |record| association = record.association(:repository) #<PullRequest> HasManyAssociation @owner = #<Repository> @loaded = true @target = [#<PullRequest>, ...]
111. Inverses @target.each do |record| association = record.association(:repository) association.target = @owner #<PullRequest> HasManyAssociation @owner = #<Repository> @loaded = true @target = [#<PullRequest>, ...]
112. Inverses @target.each do |record| association = record.association(:repository) association.target = @owner #<Repository> #<PullRequest> HasManyAssociation @owner = #<Repository> @loaded = true @target = [#<PullRequest>, ...]
113. Inverses @target.each do |record| association = record.association(:repository) association.target = @owner end Harcoded
114. Reflection @active_record = Repository @name = :pull_requests Inverses
115. #inverse_name => :repository Reflection @active_record = Repository @name = :pull_requests Inverses
116. #inverse_name => :repository Reflection @active_record = Repository @name = :pull_requests Inverses
117. Inverses class Reflection def inverse_name @active_record.name.underscore end end
118. Inverses @target.each do |record| association = record.association(:repository) association.target = @owner end Harcoded
119. Inverses @target.each do |record| association = record.association(:repository) association.target = @owner end Harcoded @reflection.inverse_name) Generic
120. Inverses > pull_requests = repository.pull_requests => SELECT * FROM pull_requests WHERE repository_id = 42
121. Inverses > pull_requests = repository.pull_requests => SELECT * FROM pull_requests WHERE repository_id = 42 > pull_requests.map(&:repository)
122. Inverses > pull_requests = repository.pull_requests => SELECT * FROM pull_requests WHERE repository_id = 42 > pull_requests.map(&:repository) 1
124. Further Studies • Through Associations, Polymorphic Associations, Scoping, Etc. • github.com/rails/rails • activerecord/lib/active_record/associations.rb • activerecord/lib/active_record/reflection.rb • activerecord/lib/active_record/associations/* • danieljamescolson.com/blog
125. Practical Applications • Use Rails features • Write less custom code • Write more efficient code • Avoid inconsistent data • Code with confidence
126. Reflecting on Active Record Associations @composerinteralia Daniel Colson @dodecadaniel