Rails Self-Referential Relationships

In a self-referential relationship a class can interact with itself using a join table. While brainstorming some ideas for my new Rails project I found the topic of self-referential has_many, throughrelationships. After reading about it I decided to try it out to see if it would fit what I wanted to do. At first it was a little confusing but after some practice with it, I found that it was very simple. I didn’t end up using this kind of relationship for my project but it was really fun to learn about it. These sort of relationships are very commonly used in social media. Some examples are followers, friendships, messages, and friend requests. I found that the messages example was the easiest to understand.

How It Works

Let’s create a users table. We want this user to be very simple for now so it will only have a name.

# db/migrate/001_create_users.rbclass

CreateUsers < ActiveRecord::Migration[6.0]
  def change
    create_table :users do |t|
      t.text :name
      t.timestamps
    end
  end
end

Next we will create the messages table. This will serve as the join table. through which the users can interact. We will give them a sender_id and a recipient_id along with the message’s content. the sender_id and recipient_id are both ids that belong to the User table.

# db/migrate/001_create_messages.rbclass

CreateMessages < ActiveRecord::Migration[6.0]
  def change
    create_table :messages do |t|
    t.text :message
    t.integer :sender_id
    t.integer :recipient_id
    t.timestamps
    end
  end
end

Now we have to work with our Message Model. We are going to tell the model that it belongs_to two both a Sender and a Recipient. Even though they come from the same class they have different foreign_key names on the messages table.

# app/models/message.rbclass

Message < ApplicationRecord
  belongs_to :recipient, class_name: "User", foreign_key: "recipient_id"
  belongs_to :sender, class_name: "User", foreign_key: "sender_id"
end

We will now set the other end of the relationship in our User model. We are telling the model that it has_many received messages and has_many sent messages.

# app/models/user.rbclass

User < ApplicationRecord
  has_many :received_messages, class_name: "Message", foreign_key: "recipient_id"
  has_many :sent_messages, class_name: "Message", foreign_key: "sender_id"end

Let’s seed the database and mess around with it.

# db/seed.rb

User.create(name: "Ray")
User.create(name: "Joe")
User.create(name: "Bob")
Message.create(message: "Hey", sender_id: User.all.sample.id, recipient_id: User.all.sample.id)
Message.create(message: "Hi", sender_id: User.all.sample.id, recipient_id: User.all.sample.id)
Message.create(message: "Yo", sender_id: User.all.sample.id, recipient_id: User.all.sample.id)

On the console.

2.6.1 :001 > ray = User.first
(0.5ms) SELECT sqlite_version(*) User Load
(0.2ms) SELECT "users".* FROM "users" ORDER BY "users"."id" ASC LIMIT ?  [["LIMIT", 1]] => 
# <User id: 1, name: "Ray", created_at: "2020-10-06 07:15:35", updated_at: "2020-10-06 07:15:35"> 2.6.1 
:002 > joe = User.secondUser Load (0.3ms)
SELECT "users".* FROM "users" ORDER BY "users"."id" ASC LIMIT ? OFFSET ?  [["LIMIT", 1], ["OFFSET", 1]] =>
# <User id: 2, name: "Joe", created_at: "2020-10-06 07:15:35", updated_at: "2020-10-06 07:15:35"> 2.6.1 :003 >
bob = User.last  User Load (0.3ms)
SELECT "users".* FROM "users" ORDER BY "users"."id" DESC LIMIT ?  [["LIMIT", 1]] =>
# <User id: 3, name: "Bob", created_at: "2020-10-06 07:15:35", updated_at: "2020-10-06 07:15:35">

User’s received messages.

2.6.1 :012 > ray.received_messages.first.message
Message Load (0.2ms)  SELECT "messages".* FROM "messages" WHERE "messages"."recipient_id" = ? ORDER BY "messages"."id" ASC LIMIT ? [["recipient_id", 1], ["LIMIT", 1]] =>
"Hey" 2.6.1 :013 > bob.received_messages.first.message  Message Load (0.2ms)  SELECT "messages".* FROM "messages"
WHERE "messages"."recipient_id" = ? ORDER BY
"messages"."id" ASC LIMIT ?  [["recipient_id", 3], ["LIMIT", 1]] => "Yo" 2.6.1 :014 > joe.received_messages.first.message  Message Load (0.2ms)  SELECT "messages".* FROM "messages" WHERE "messages"."recipient_id" = ? ORDER BY "messages"."id" ASC LIMIT ?  [["recipient_id", 2], ["LIMIT", 1]] => "Hi"

User’s sent messages.

2.6.1 :020 > bob.sent_messages.first.message  Message Load (0.2ms)  SELECT "messages".* FROM "messages" WHERE "messages"."sender_id" = ? ORDER BY "messages"."id" ASC LIMIT ?  [["sender_id", 3], ["LIMIT", 1]] => "Hi" 2.6.1 :021 > joe.sent_messages.first.message  Message Load (0.2ms)  SELECT "messages".* FROM "messages" WHERE "messages"."sender_id" = ? ORDER BY "messages"."id" ASC LIMIT ?  [["sender_id", 2], ["LIMIT", 1]] => "Hey" 2.6.1 :022 >

That is all! Make sure to mess with different kinds of models so that it is a little bit easier to understand. I am sure there are many ways to have fun with this relationship. Happy coding! 😎