Associations in Rails

Associations Modeling database relationships with ActiveRecord

Database Modeling The “relational” in relational database

Database Types

Database Types Rails was designed with a relational database bias

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

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

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

Relationship Types

Relationship Types There are only three types of relationships in SQL

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

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

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

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

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

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!

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

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

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

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

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

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!

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!

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!

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

Rails Macros

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

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

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

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

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

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!

One to Many An article has_many() comments

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

Associated Models You can query through the association

>> 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

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

>> 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

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

>> 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()

Using Associations Correctly

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

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

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

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

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

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

One to One A user has_one() photo

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

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

>> 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())

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

Many to Many Photos have many tags

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

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

>> 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

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

Many to Many (Two Tables) Users have many followers

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

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

>> 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

Extras Rails associations have quite a few options

Finder Options and Dependance

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

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

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

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()

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

Polymorphic Associations

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

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

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

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)

Questions?

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

by James Gray on Mar 27, 2010

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