relationships

Last updated 7 months ago
One to One
For one-to-one relationships, You can use the has_one and belongs_to in your models.
Note: one-to-one relationship does not support through associations yet.
class Team < Granite::Base
  has_one :coach
​
  field name : String
end
This will add a coach and coach= instance methods to the team which returns associated coach.
class Coach < Granite::Base
  table_name :coaches
​
  belongs_to :team
​
  field name : String
end
This will add a team and team= instance method to the coach.
For example:
team = Team.find 1
# has_one side..
puts team.coach
​
coach = Coach.find 1
# belongs_to side...
puts coach.team
​
coach.team = team
coach.save
​
# or in one-to-one you can also do
​
team.coach = coach
# coach is the child entity and contians the foreign_key
# so save should called on coach instance
coach.save
In this example, you will need to add a team_id and index to your coaches table:
CREATE TABLE coaches (
  id BIGSERIAL PRIMARY KEY,
  team_id BIGINT,
  name VARCHAR,
  created_at TIMESTAMP,
  updated_at TIMESTAMP
);
​
CREATE INDEX 'team_id_idx' ON coaches (team_id);
Foreign key is inferred from the class name of the Model which uses has_one. In above case team_id is assumed to be present in coaches table. In case its different you can specify one like this:
class Team < Granite::Base
  has_one :coach, foreign_key: :custom_id
​
  field name : String
end
​
class Coach < Granite::Base
  belongs_to :team
end
The class name inferred from the name but you can specify the class name:
class Team < Granite::Base
  has_one coach : Coach, foreign_key: :custom_id
​
  # or you can provide the class name as a parameter
  has_one :coach, class_name: Coach, foreign_key: :custom_id
​
  field name : String
end
​
class Coach < Granite::Base
  belongs_to team : Team
​
  # provide a custom foreign key
  belongs_to team : Team, foreign_key: team_uuid : String
end
One to Many
belongs_to and has_many macros provide a rails like mapping between Objects.
class User < Granite::Base
  adapter mysql
​
  has_many :post
​
  # pluralization requires providing the class name
  has_many posts : Post
​
  # or you can provide class name as a parameter
  has_many :posts, class_name: Post
​
  # you can provide a custom foreign key
  has_many :posts, class_name: Post, foreign_key: :custom_id
​
  field email : String
  field name : String
  timestamps
end
This will add a posts instance method to the user which returns an array of posts.
class Post < Granite::Base
  adapter mysql
  table_name :posts
​
  belongs_to :user
​
  # or custom name
  belongs_to my_user : User
​
  # or custom foreign key
  belongs_to user : User, foreign_key: uuid : String
​
  field title : String
  timestamps
end
This will add a user and user= instance method to the post.
For example:
user = User.find 1
user.posts.each do |post|
  puts post.title
end
​
post = Post.find 1
puts post.user
​
post.user = user
post.save
In this example, you will need to add a user_id and index to your posts table:
CREATE TABLE posts (
  id BIGSERIAL PRIMARY KEY,
  user_id BIGINT,
  title VARCHAR,
  created_at TIMESTAMP,
  updated_at TIMESTAMP
);
​
CREATE INDEX 'user_id_idx' ON posts (user_id);
Many to Many
Instead of using a hidden many-to-many table, Granite recommends always creating a model for your join tables. For example, let's say you have many users that belong to many rooms. We recommend adding a new model called participants to represent the many-to-many relationship.
Then you can use the belongs_to and has_many relationships going both ways.
class User < Granite::Base
  has_many :participants, class_name: Participant
​
  field name : String
end
​
class Participant < Granite::Base
  table_name :participants
​
  belongs_to :user
  belongs_to :room
end
​
class Room < Granite::Base
  table_name :rooms
​
  has_many :participants, class_name: Participant
​
  field name : String
end
The Participant class represents the many-to-many relationship between the Users and Rooms.
Here is what the database table would look like:
CREATE TABLE participants (
  id BIGSERIAL PRIMARY KEY,
  user_id BIGINT,
  room_id BIGINT,
  created_at TIMESTAMP,
  updated_at TIMESTAMP
);
​
CREATE INDEX 'user_id_idx' ON TABLE participants (user_id);
CREATE INDEX 'room_id_idx' ON TABLE participants (room_id);
has_many through:
As a convenience, we provide a through: clause to simplify accessing the many-to-many relationship:
class User < Granite::Base
  has_many :participants, class_name: Participant
  has_many :rooms, class_name: Room, through: :participants
​
  field name : String
end
​
class Participant < Granite::Base
  belongs_to :user
  belongs_to :room
end
​
class Room < Granite::Base
  has_many :participants, class_name: Participant
  has_many :users, class_name: User, through: :participants
​
  field name : String
end
This will allow you to find all the rooms that a user is in:
user = User.first
user.rooms.each do |room|
  puts room.name
end
And the reverse, all the users in a room:
room = Room.first
room.users.each do |user|
  puts user.name
end
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