Question 3

20150923034545_create_people.rb

class CreatePeople < ActiveRecord::Migration
  def change
    create_table :people do |t|
      t.string :name
      t.references :parent, index: true
      t.timestamps null: false
    end
  end
end

Answer.txt

1 . Chỉ cần column People cho việc thực hiện migration tạo bảng cho Person.
2. update model Person:

class Person < ActiveRecord::Base
  belongs_to :parent, class: Person
  has_many :children, class: Person, foreign_key: :parent_id
  has_many :grandchildren, class: Person, through: :children, source: :children
end

Person.rb

class Person < ActiveRecord::Base
  has_many :children, class_name: Person, foreign_key: :parent_id
 
  belongs_to :parent, class_name: Person
end