Active Record Associations Overview

ar-relations.md

Active Record Associations

Overview

• belongs_to
• has_many
• has_many :through
• ActiveRecord::Relation
• Remember: Models are not Migrations

Rails Guides

• Associations
• Migrations
• Associations Class Methods

Our Model

• Album belongs_to Artist
• Artist has_many Albums
• Track belongs_to Album
• Albums has_many Tracks
• Artist has_many Tracks :through Albums

Convention vs Configuration

Most of the time we can use the conventions Sometimes we need to customize

• tablename <- Table Name
• :foreign_key <- Foreign Key Column Name
• :class_name <- Class of Associated Type

Option to Cut and Run

New Release...

PM: Artists sometimes help by producing an Album by a different Artist

• What is the new relationship?
• How could we express it in Active Record?
• What changes do we need to make to the schema?
• What is our work flow to change the schema for a published app?

Didn't Talk about

• has_and_belongs_to_many Join table with no data

Helpful Code

• Pro Tip: ActiveRecord::Relation#to_sql
• ActiveRecord::Base.logger = Logger.new(STDOUT)
• autoload :Artist, APP_ROOT.join('app', 'models', 'artist')
• Migration Filename : Time.now.utc.strftime("%Y%m%d%H%M%S")
• Look into https://github.com/janko-m/sinatra-activerecord
• $ sqlite3 db/ar-relations.sqlite3
• Track.new.methods - ActiveRecord::Base.instance_methods
• new Artist: acdc = Artist.create(:name => 'AC/DC')
• new Album: h2h = acdc.albums.create(title: 'Highway to Hell')
• new Track: t2m = h2h.tracks.create(title: 'Touch Too Much')
• Artist from Track: delegate :artist, :to => :album, :allow_nil => true
• acdc.albums.to_sql
• acdc.tracks.to_sql
• Interesting: acdc = Artist.find_or_create_by(:name => 'AC/DC')