Relations

Relations is a Active Record queries collection based on Rails 4 and Ruby 2

The magic happens within the migrations, the models and db/seed.rb.

To test the examples just run:

$ rake db:setup

Self-referential one-to-one relationship

Everybody has one mother. At most.

$ rails g model user first_name:string mother_id:integer
$ rake db:migrate

User model

has_one :mother, class_name: 'User', foreign_key: 'mother_id'

db/seed

john = User.create(first_name: 'John')
heidi = User.create(first_name: 'Heidi')

john.mother = heidi

puts "John's mother is " + Rainbow("#{john.mother.first_name}").yellow

=> John's mother is Heidi

Join model many-to-many relationship

A user wants to add all videos he likes to a playlist.

$ rails g model video title:string engine:string duration:integer
$ rails g model playlist name:string user_id:integer published:boolean
$ rails g model like video_id:integer playlist_id:integer
$ rake db:migrate

Playlist model

belongs_to :user
has_many :likes
has_many :videos, :through => :likes

User model

has_many :playlists
has_many :likes, :through => :playlists

Like model

belongs_to :video
belongs_to :playlist

db/seed

animals = Video.create([
  {title: 'Cat', engine: 'youtube', duration: 90},
  {title: 'Dog', engine: 'youtube', duration: 120}])

# to create a user playlist we use the create method of the association proxy
playlist = john.playlists.create name: 'Animals'
# the foreign keys will be automatically set and the objects will be automatically saved by the << method
playlist.likes << animals.map{|animal| Like.create video: animal}

puts playlist.videos.count

=> 2

fruits = Video.create([
  {title: 'Banana', engine: 'vimeo', duration: 140},
  {title: 'Apple', engine: 'dailymotion', duration: 240},
  {title: 'Orange', engine: 'dailymotion', duration: 30}])

playlist = john.playlists.create name: 'Fruits'
# the << method takes both a single associated object or an array of objects
playlist.likes << fruits.map{|fruit| Like.create video: fruit}

puts playlist.videos.count

=> 3

john.likes.each{|like| puts like.video.title}

=> Cat
=> Dog
=> Banana
=> Apple
=> Orange

Inspired by Clipflakes' video search engine and playlist creation.

Order a has_many through association

Now we want a sorted list of all videos John likes. The list has to be sorted by a given video attribute, e.g. the name of the engine or the duration. Instead of iterate through all Like's we want the video list through an additional has_many through association. Example:

videos = john.videos.sort(:engine)

User model

has_many :videos, :through => :likes

Video model

scope :sort, ->(column) { order column }

Videos sorted by title, engine and duration:

db/seed

john.videos.sort(:title).each{|video| 
  puts Rainbow("#{video.title}").yellow + " from #{video.engine.titleize} has a duration of #{video.duration} seconds"}

john.videos.sort(:engine).each{|video| 
  puts Rainbow("#{video.engine.titleize}").yellow + "'s #{video.title} has a duration of #{video.duration} seconds"}

john.videos.sort(:duration).each{|video| 
  puts Rainbow("#{video.duration}").yellow + " seconds is the duration of #{video.title} from #{video.engine.titleize}"}

=> Apple from Dailymotion has a duration of 240 seconds
=> Banana from Vimeo has a duration of 140 seconds
=> Cat from Youtube has a duration of 90 seconds
=> Dog from Youtube has a duration of 120 seconds
=> Orange from Dailymotion has a duration of 30 seconds

=> Dailymotion Apple has a duration of 240 seconds
=> Dailymotion Orange has a duration of 30 seconds
=> Vimeo Banana has a duration of 140 seconds
=> Youtube Cat has a duration of 90 seconds
=> Youtube Dog has a duration of 120 seconds

=> 30 seconds is the duration of Orange from Dailymotion
=> 90 seconds is the duration of Cat from Youtube
=> 120 seconds is the duration of Dog from Youtube
=> 140 seconds is the duration of Banana from Vimeo
=> 240 seconds is the duration of Apple from Dailymotion

The ActiveRecord::Base order method works the same. It's one of the association collection wrapper methods.

john.videos.order(:title).each{|video| 
  puts Rainbow("#{video.title}").yellow + " from #{video.engine.titleize} has a duration of #{video.duration} seconds"}

=> Apple from Dailymotion has a duration of 240 seconds
=> Banana from Vimeo has a duration of 140 seconds
=> Cat from Youtube has a duration of 90 seconds
=> Dog from Youtube has a duration of 120 seconds
=> Orange from Dailymotion has a duration of 30 seconds

Inspired by the article Sorting and Reordering Has Many Through Associations With the ActsAsList Gem.

Combined scope conditions

While the sort scope in the previous example was just another way to order a query result we now want a new scope condition for returning only videos with a given minimum length.

Video model

scope :duration_min, ->(seconds) { where(duration: seconds.to_i..Float::INFINITY) }

Scopes can be combined: display all videos with at least 100 seconds ordered by their engine names:

db/seed

john.videos.duration_min(100).sort(:engine).each{|video| 
  puts Rainbow("#{video.engine.titleize}").yellow + " #{video.title} has a duration of #{video.duration} seconds"}

=> Dailymotion Apple has a duration of 240 seconds
=> Vimeo Banana has a duration of 140 seconds
=> Youtube Dog has a duration of 120 seconds

Merging scopes with joins

We now want all videos from a specific playlist. We can do this by combining a join with a merge condition.

Video model

has_many :likes
has_many :playlist, :through => :likes
scope :list, ->(name) { joins(:playlist).merge( Playlist.where(name: name) )}

db/seed

%w[Animals Fruits].each do |name|
  # use of association collection average method
  puts "Playlist " + Rainbow("#{name}").yellow + " has an average duration of " + Rainbow("#{john.videos.list(name).average(:duration).to_i}").yellow + " seconds"
  john.videos.list(name).each{|video| 
  puts "Playlist " + Rainbow("#{name}").yellow + " has video " + Rainbow("#{video.title}").yellow + " from #{video.engine.titleize}"}
end

=> Playlist Animals has an average duration of 105 seconds
=> Playlist Animals has video Cat from Youtube
=> Playlist Animals has video Dog from Youtube
=> Playlist Fruits has an average duration of 136 seconds
=> Playlist Fruits has video Banana from Vimeo
=> Playlist Fruits has video Apple from Dailymotion
=> Playlist Fruits has video Orange from Dailymotion

Merge can also call a scope

Let's search for all playlists that have at least one Vimeo video.

Video model

scope :vimeo, -> { where(engine: 'vimeo') }

Playlist model

scope :vimeo, -> { joins(:videos).merge( Video.vimeo ) }

db/seed

# how many videos from Vimeo do we have?
puts Video.vimeo.count
# display the title of the first Vimeo video in the first playlist that has at least one video from Vimeo
puts john.playlists.vimeo.first.videos.vimeo.first.title

#=> 1
#=> Banana

Inspired by Railscasts '#215 Advanced Queries in Rails 3'