Users - Followers - Following

migration.rb

class CreateRelations < ActiveRecord::Migration
  def change
    create_table :relations do |t|
      t.references :user, index: true, foreign_key: true
      t.integer :friend_id, index: true
      t.timestamps null: false
    end
  end
end

relation.rb

class Relation < ActiveRecord::Base
  belongs_to :user
  belongs_to :friend, class_name: "User"

  validates_uniqueness_of :user_id, scope: :friend_id
end

user.rb

class User < ActiveRecord::Base
  has_many :follower_relations, class_name: "Relation", foreign_key: :friend_id
  has_many :following_relations, class_name: "Relation", foreign_key: :user_id
  has_many :followers, through: :follower_relations, source: :user
  has_many :followings, through: :following_relations, source: :friend
end