[Ruby on Rails] SubModel in Model

Category
  Sub Category
  Sub Category
  Sub Category

migrate.rb

class CreateCategories < ActiveRecord::Migration[5.0]
  def change
    create_table :categories do |t|
      t.string :name
      t.references :parent

      t.timestamps
    end
  end
end

model.rb

class Category < ApplicationRecord
  has_many :categories, class_name: 'Category', foreign_key: 'parent_id',
    dependent: :destroy
  belongs_to :parent, class_name: 'Category', optional: true
end

c.parent
c.categories