Associations in Rails

1. Associations Modeling database relationships with ActiveRecord

2. Database Modeling The “relational” in relational database

3. Database Types

4. Database Types Rails was designed with a relational database bias

5. Database Types Rails was designed with a relational database bias Though it can work with other tools, we will stick with this primary focus

6. Database Types Rails was designed with a relational database bias Though it can work with other tools, we will stick with this primary focus ActiveRecord can work with the SQL these databases use to make queries

7. Database Types Rails was designed with a relational database bias Though it can work with other tools, we will stick with this primary focus ActiveRecord can work with the SQL these databases use to make queries One of the ways it does that is to manage the relationships of data in separate tables

8. Relationship Types

9. Relationship Types There are only three types of relationships in SQL

10. Relationship Types There are only three types of relationships in SQL One to one

11. Relationship Types There are only three types of relationships in SQL One to one One to many

12. Relationship Types There are only three types of relationships in SQL One to one One to many Many to many

13. Relationship Types There are only three types of relationships in SQL One to one One to many Many to many This doesn’t really exist, but it can be simulated

14. One to One users id ﬁrst_name last_name 1 James Gray 2 Dana Gray photos id user_id path 1 1 /images/james.png

16. One to Many articles id title body comments 1 The CRUD … id article_id body 2 Associations … 1 1 First! 2 1 Tard. 3 1 Am not! 4 2 Viagra 5 2 Spam!

19. photos Many to Many id path 1 /images/mac.jpg taggings 2 /images/pc.bmp id photo_id tag_id 1 1 1 tags id name 2 1 2 1 computer 3 1 3 2 sexy 4 2 4 3 lickable 4 junk

22. Many to Many (Two Tables) followings users id follower_id followed_id id ﬁrst_name last_name 1 1 4 1 James Gray 2 2 4 2 Dana Gray 3 3 1 3 Super Man 4 4 1 4 Bat Man 5 5 1 5 Spider Man

25. Conventions at Work articles id title body 1 Unique … comments id article_id body 1 1 First!

26. Conventions at Work Rails favors conventions articles id title body 1 Unique … comments id article_id body 1 1 First!

27. Conventions at Work Rails favors conventions articles With associations: id title body 1 Unique … comments id article_id body 1 1 First!

28. Conventions at Work Rails favors conventions articles With associations: id title body Auto-incremented ID 1 Unique … ﬁelds comments id article_id body 1 1 First!

29. Conventions at Work Rails favors conventions articles With associations: id title body Auto-incremented ID 1 Unique … ﬁelds Table name is plural comments for the collection id article_id body 1 1 First!

30. Conventions at Work Rails favors conventions articles With associations: id title body Auto-incremented ID 1 Unique … ﬁelds Table name is plural comments for the collection id article_id body Foreign key is a singular item plus _id 1 1 First!

31. Associations, Rails Style Let’s turn those examples into real Rails code

32. Rails Macros

33. Rails Macros Use belongs_to() on the ID side of a one to one or one to many

34. Rails Macros Use belongs_to() on the ID side of a one to one or one to many Use has_one() on the other side of a one to one

35. Rails Macros Use belongs_to() on the ID side of a one to one or one to many Use has_one() on the other side of a one to one Use has_many() on the other side of a one to many

36. Rails Macros Use belongs_to() on the ID side of a one to one or one to many Use has_one() on the other side of a one to one Use has_many() on the other side of a one to many Use two belongs_to() calls for the join model of a many to many

37. Rails Macros Use belongs_to() on the ID side of a one to one or one to many Use has_one() on the other side of a one to one Use has_many() on the other side of a one to many Use two belongs_to() calls for the join model of a many to many Use has_many() and has_many(…, :through => …) for the data models of a many to many

39. One to Many An article has_many() comments

40. class Article < ActiveRecord::Base has_many :comments end class Comment < ActiveRecord::Base belongs_to :article end One to Many An article has_many() comments

41. Associated Models You can query through the association

42. >> crud = Article.ﬁrst => #<Article id: 1, title: "The CRUD", body: "...", …> >> crud.comments.count => 3 >> crud.comments => [#<Comment id: 1, article_id: 1, body: "First!", …>, #<Comment id: 2, article_id: 1, body: "Tard.", …>, #<Comment id: 3, article_id: 1, body: "Am not!", …>] >> crud.comments.last => #<Comment id: 3, article_id: 1, body: "Am not!", …> >> crud.comments.last.article => #<Article id: 1, title: "The CRUD", body: "...", …> Associated Models You can query through the association

47. Adding to an Association There are multiple ways to add in new models

48. >> a = Article.create!(:title => "Needs Comments") => #<Article id: 3, title: "Needs Comments", body: nil, …> >> a.comments => [] >> a.comments.create!(:body => "create!()") => #<Comment id: 6, article_id: 3, body: "create!()", …> >> a.comments << Comment.new(:body => "<<") => [#<Comment id: 6, article_id: 3, body: "create!()", …>, #<Comment id: 7, article_id: 3, body: "<<", …>] >> c = a.comments.build(:body => "build()") => #<Comment id: nil, article_id: 3, body: "build()", created_at: nil, updated_at: nil> >> c.save! => true >> a.comments => [#<Comment id: 6, article_id: 3, body: "create!()", …>, #<Comment id: 7, article_id: 3, body: "<<", …>, #<Comment id: 8, article_id: 3, body: "build()", …>] Adding to an Association There are multiple ways to add in new models

52. Deleting Associated Models To delete associated models, ﬁnd normally and call destroy()

53. >> a.comments => [#<Comment id: 6, article_id: 3, body: "create!()", …>, #<Comment id: 7, article_id: 3, body: "<<", …>, #<Comment id: 8, article_id: 3, body: "build()", …>] >> c = a.comments.ﬁnd(8) => #<Comment id: 8, article_id: 3, body: "build()", …> >> c.destroy if c => #<Comment id: 8, article_id: 3, body: "build()", …> >> a.comments(true) => [#<Comment id: 6, article_id: 3, body: "create!()", …>, #<Comment id: 7, article_id: 3, body: "<<", …>] Deleting Associated Models To delete associated models, ﬁnd normally and call destroy()

57. Using Associations Correctly

58. Using Associations Correctly You should very rarely need to refer to an ID

59. Using Associations Correctly You should very rarely need to refer to an ID Let Rails worry about that for you

60. Using Associations Correctly You should very rarely need to refer to an ID Let Rails worry about that for you It’s safer to scope as many queries as possible through the association

61. Using Associations Correctly You should very rarely need to refer to an ID Let Rails worry about that for you It’s safer to scope as many queries as possible through the association There’s less chance to affect the wrong data

62. Using Associations Correctly You should very rarely need to refer to an ID Let Rails worry about that for you It’s safer to scope as many queries as possible through the association There’s less chance to affect the wrong data It adds security against users trying to spoof ID’s

64. One to One A user has_one() photo

65. class User < ActiveRecord::Base has_one :photo end class Photo < ActiveRecord::Base belongs_to :user end One to One A user has_one() photo

66. Model Attributes Just assign a belongs_to()/has_one() to set it (or call build_photo()/create_photo())

67. >> james = User.find_by_first_name("James") => #<User id: 1, first_name: "James", last_name: "Gray", …> >> james.photo = Photo.new(:path => "/images/james.png") => #<Photo id: 1, user_id: 1, path: "/images/james.png", …> Model Attributes Just assign a belongs_to()/has_one() to set it (or call build_photo()/create_photo())

68. >> james = User.find_by_first_name("James") => #<User id: 1, first_name: "James", last_name: "Gray", …> >> james.photo = Photo.new(:path => "/images/james.png") => #<Photo id: 1, user_id: 1, path: "/images/james.png", …> Model Attributes Just assign a belongs_to()/has_one() to set it (or call build_photo()/create_photo())

70. Many to Many Photos have many tags

71. class Photo < ActiveRecord::Base has_many :taggings has_many :tags, :through => :taggings end class Tagging < ActiveRecord::Base belongs_to :photo belongs_to :tag end class Tag < ActiveRecord::Base has_many :taggings has_many :photos, :through => :taggings end Many to Many Photos have many tags

72. class Photo < ActiveRecord::Base has_many :taggings has_many :tags, :through => :taggings end class Tagging < ActiveRecord::Base belongs_to :photo belongs_to :tag end class Tag < ActiveRecord::Base has_many :taggings has_many :photos, :through => :taggings end Many to Many Photos have many tags

73. has_many :through The nested association is mostly managed by Rails for you

74. >> mac = Photo.ﬁrst => #<Photo id: 1, user_id: 1, path: "/images/mac.jpg", …> >> mac.taggings => [#<Tagging id: 1, photo_id: 1, tag_id: 1, …>, #<Tagging id: 2, photo_id: 1, tag_id: 2, …>, #<Tagging id: 3, photo_id: 1, tag_id: 3, …>] >> mac.tags => [#<Tag id: 1, name: "computer", …>, #<Tag id: 2, name: "sexy", …>, #<Tag id: 3, name: "lickable", …>] >> pc = Photo.last => #<Photo id: 2, user_id: nil, path: "/images/pc.bmp", …> >> comp = Tag.ﬁnd_or_create_by_name("computer") => #<Tag id: 1, name: "computer", …> >> pc.taggings.create!(:tag => comp) => #<Tagging id: 5, photo_id: 2, tag_id: 1, …> has_many :through The nested association is mostly managed by Rails for you

79. Many to Many (Two Tables) Users have many followers

80. class Following < ActiveRecord::Base belongs_to :follower, :class_name => "User", :foreign_key => :follower_id belongs_to :followed, :class_name => "User", :foreign_key => :followed_id end class User < ActiveRecord::Base has_many :other_followings, :class_name => "Following", :foreign_key => :followed_id has_many :followers, :through => :other_followings has_many :self_followings, :class_name => "Following", :foreign_key => :follower_id has_many :followed_by, :through => :self_followings, :source => :followed end Many to Many (Two Tables) Users have many followers

84. Two-way Relationships Once you get the relationships right, Rails can still manage the messy details

85. >> james = User.find_by_first_name("James") => #<User id: 1, first_name: "James", last_name: "Gray", …> >> james.followers => [#<User id: 3, first_name: "Super", last_name: "Man", …>, #<User id: 4, first_name: "Bat", last_name: "Man", …>, #<User id: 5, first_name: "Spider", last_name: "Man", …>] >> james.followed_by => [#<User id: 4, first_name: "Bat", last_name: "Man", …>] >> bat = User.find_by_first_name("Bat") => #<User id: 4, first_name: "Bat", last_name: "Man", …> >> bat.followers => [#<User id: 1, first_name: "James", last_name: "Gray", …>, #<User id: 2, first_name: "Dana", last_name: "Gray", …>] Two-way Relationships Once you get the relationships right, Rails can still manage the messy details

88. Extras Rails associations have quite a few options

89. Finder Options and Dependance

90. Finder Options and Dependance Set :conditions, :order, etc. on associations

91. Finder Options and Dependance Set :conditions, :order, etc. on associations Example: :order => “created_at DESC”

92. Finder Options and Dependance Set :conditions, :order, etc. on associations Example: :order => “created_at DESC” Specify what happens to associations with destroy()

93. Finder Options and Dependance Set :conditions, :order, etc. on associations Example: :order => “created_at DESC” Specify what happens to associations with destroy() :dependent => :destroy forwards destroy()

94. Finder Options and Dependance Set :conditions, :order, etc. on associations Example: :order => “created_at DESC” Specify what happens to associations with destroy() :dependent => :destroy forwards destroy() :dependent => :nullify breaks the link

95. Polymorphic Associations

96. Polymorphic Associations Rails has a special type of “Polymorphic Association”

97. Polymorphic Associations Rails has a special type of “Polymorphic Association” Models are linked with a type and an ID

98. Polymorphic Associations Rails has a special type of “Polymorphic Association” Models are linked with a type and an ID This makes it possible to link one model to multiple different kinds of other models

99. Polymorphic Associations Rails has a special type of “Polymorphic Association” Models are linked with a type and an ID This makes it possible to link one model to multiple different kinds of other models For example, you might use a Comment model to allows users to comment on multiple things (products and reviews for example)

100. Questions?

101. Fleshing out the Data Layer Lab Your book has instructions on how to add new models and associate them with existing tables

Associations in Rails

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