ActiveRecord Associations

Chennai-Rails Workshop 

21 March 2015

Abhishek Yadav

@h6165

Database Relationships

one-many

one-one

many-many

One to many

Team

Player

team_id​

One to many

Team 
  has_many :players

• We declare in Team model
• Rails guesses it refers to the Player model
• Rails guesses there's a 'team_id' in 'players' table 

One to many

Team has_many :players

team.players = Player.find(2,3,4)
team.players 
team.players.create(name: 'Abhi') 
team.players.where(name: 'Abhi') 
team.players.destroy

So we can do:

One to many

Player belongs_to :team

So we can do:

• We declare in Player model
• Rails guesses it refers to the Team model
• Rails guesses there's a 'team_id' in 'teams' table 

player.team
player.team = Team.last 

One to one

Team

Coach

team_id

One to one

Team
  has_one :coach

• We declare in Team model
• Rails guesses it refers to the Coach model
• Rails guesses there's a 'team_id' in 'coaches' table 

One to one

Team
  has_one :coach

We can do -

team.coach

team.coach = Coach.find (2)

team.coach.create(name: 'Steve')

Many to many

Person

Match

Person watches many matches

Match has many people watching

Many to many

Person watches many matches

Match has many people watching

Should have a join table

matches_people

person_id

match_id

Many to many

Person

        has_many :matches, through: :matches_people

        has_many :matches_people

Match

        has_many :people, through: :matches_people

        has_many :matches_people

MatchesPerson

        belongs_to :matches

        belongs_to :person

has_many :through

Many to many

Person
     has_many :matches, through: :matches_people
     has_many :matches_people
Match
     has_many :persons, through: :matches_people
     has_many :matches_people

• Rails figures that there is a join table called matches_people
• Rails assumes it has person_id and match_id fields in it.

Many to many

Person
     has_many :matches, through: :matches_people
     has_many :matches_people
Match
     has_many :persons, through: :matches_people
     has_many :matches_people

We can do:

match.persons = Person.find(1,2,3)

match.persons << Person.find(4,5,6)

person.matches << Match.where(team1: 'India')

Many to many

Person
     has_and_belongs_to_many :matches
Match
     has_and_belong_to_many :people

• Join table matches_persons should be created separately
• Join table is not a model

has_and_belongs_to_many

More:

http://guides.rubyonrails.org/association_basics.html