Question

In RoR3,

I have Users and Skills and each skill is created by a user. I wanted to record that, so I created a one to many relationship.

class User < ActiveRecord::Base
  has_many :skills
end

class Skill < ActiveRecord::Base
  belongs_to :user
end

However, each user also has many skills in the sense that, user “Bob” created skill “Kung Fu”, user “Charlie” created skill “Karate” and user “Bob” both created and is able to do both “Kung Fu” and “Karate”

How should I represent this with ActiveRecord? Should I just create a new table “user_skills” which has_many :skills? and belong_to :user?

Answer 1

There are two different associations here. The first is a one-to-many association. An user can be the creator of any number of skills. The second one is a many-to-many association, an user can have many skills and a skill can have many users.

The first one is a simple belongs_to <-> has_many declaration. For the second one, you either need a has_and_belongs_to_many declaration in both models, and a related join table, or a dedicated join model, and a has_many :through declaration. Let’s try the first one:

Method 1: HABTM

class User < ActiveRecord::Base
  has_many :created_skills, :class_name => 'Skill', :inverse_of => :creator
  has_and_belongs_to_many :skills
end

class Skill < ActiveRecord::Base
  belongs_to :creator, :class_name => 'User', :inverse_of => :created_skills
  has_and_belongs_to_many :users
end

This requires a join table called “skills_users” that has columns named user_id and skill_id

Method 2: Has many through (Join model)

The second one is similar, but adds a model that acts as the middleman. This has an added benefit that you can include additional columns in the join model, like for example a skill level.

class User < ActiveRecord::Base
  has_many :created_skills, :class_name => 'Skill', :inverse_of => :creator
  has_many :user_skills
  has_many :skills, :through => :user_skills
end

class Skill < ActiveRecord::Base
  belongs_to :creator, :class_name => 'User', :inverse_of => :created_skills
  has_many :user_skills
  has_many :users, :through => :user_skills
end

class UserSkill < ActiveRecord::Base
  belongs_to :user
  belongs_to :skill
end