Many-to-many with has_many through in Ruby on Rails

Introduction

This pattern helps connect two models with a middle model, so they can relate to each other many times.

You want to link students and courses where each student can join many courses and each course has many students.

You need to track extra details about the connection, like when a user liked a post.

You want to organize books and authors where each book can have many authors and each author can write many books.

Syntax

Ruby on Rails

class ModelA < ApplicationRecord
  has_many :join_models
  has_many :model_bs, through: :join_models
end

class JoinModel < ApplicationRecord
  belongs_to :model_a
  belongs_to :model_b
end

class ModelB < ApplicationRecord
  has_many :join_models
  has_many :model_as, through: :join_models
end

The join model is a separate table that connects the two main models.

Use has_many :through to tell Rails to go through the join model.

Examples

This example connects students and courses through enrollments.

Ruby on Rails

class Student < ApplicationRecord
  has_many :enrollments
  has_many :courses, through: :enrollments
end

class Enrollment < ApplicationRecord
  belongs_to :student
  belongs_to :course
end

class Course < ApplicationRecord
  has_many :enrollments
  has_many :students, through: :enrollments
end

This example connects authors and books through book_authors.

Ruby on Rails

class Author < ApplicationRecord
  has_many :book_authors
  has_many :books, through: :book_authors
end

class BookAuthor < ApplicationRecord
  belongs_to :author
  belongs_to :book
end

class Book < ApplicationRecord
  has_many :book_authors
  has_many :authors, through: :book_authors
end

Sample Program

This code sets up users and groups connected through memberships. A user can join many groups, and a group can have many users.

Ruby on Rails

class User < ApplicationRecord
  has_many :memberships
  has_many :groups, through: :memberships
end

class Membership < ApplicationRecord
  belongs_to :user
  belongs_to :group
end

class Group < ApplicationRecord
  has_many :memberships
  has_many :users, through: :memberships
end

# Usage example in Rails console:
# user = User.create(name: "Alice")
# group = Group.create(name: "Chess Club")
# Membership.create(user: user, group: group)
# user.groups.pluck(:name) # => ["Chess Club"]

Important Notes

The join model can store extra info about the connection, like join date.

Always add belongs_to in the join model for both connected models.

Summary

Use has_many :through to connect two models with a join model.

This allows many-to-many relationships with extra data on the connection.

It keeps your data organized and easy to query.