Twitter Follows

Objectives

Create a complex Domain Model
Implement an active follow relationship (following )
Implement the passive relationship (i.e. followers )

Instructions

In Twitter, a user follows many other users. That person can be followed by many people. For example, I follow @coffeedad and @cher. @coffeedad has many followers and so does @cher.

How you would you domain model this out?

A user can either follow or be followed by another user. Since this relationship can be true of many users, we have a many-to-many relationship between users. Consequently, we need at least two models: the User model and a join model. Creating another model for users (like a followers model) would be redundant, as it would replicate the information present in the User model. A user has many other users through relationships, for a total of two models.

What tables/foreign keys would you need?

We determined previously that we need two models. In Active Record, each model corresponds to a table. We will have a Users table and a Relationships table.

class User < ActiveRecord::Migration[5.2]
  def change
    create_table :users do |t|
      t.string :username
      t.timestamps
    end
  end
end

For the sake of this exercise, we will keep the attributes of the User table simple, with just a username.

class CreateRelationships < ActiveRecord::Migration[5.2]
  def change
    create_table :relationships do |t|
      t.integer :follower_id
      t.integer :followed_id
      t.timestamps
    end
  end
end

The Relationships table introduces more interesting elements, with us specifying a columns for the follower and followed user ids.

How would you domain model this out?

class Relationship < ActiveRecord::Base
  belongs_to :followed, class_name: "User", foreign_key: :followed_id
  belongs_to :follower, class_name: "User", foreign_key: :follower_id
end

In defining our ActiveRecord class, we begin implementing a self join. We must specify the name of the class/model to which the relationship belongs, as well as the foreign key.

class User < ActiveRecord::Base
  has_many :active_relationships, class_name: "Relationship", foreign_key: :follower_id, dependent: :destroy
  has_many :followeds, through: :active_relationships, source: :followed
  has_many :passive_relationships, class_name: "Relationship", foreign_key: :followed_id, dependent: :destroy
  has_many :followers, through: :passive_relationships, source: :follower
end

We close out the definition of this relationship by implementing our many-to-many self join of users. Like with the Relationship model, we create an alias/method for the relationship between users.

Resources

Rails Tutorial - Following Users

View this lesson on Learn.co

bjf-flatiron / twitter-follows-readme