doctor.rb

require 'sqlite3'
DB = SQLite3::Database.new('test.db')

class Doctor
  attr_reader :id

  def initialize(attributes = {})
    @id = attributes[:id]
    @name = attributes[:name]
    @age = attributes[:age]
    @specialty = attributes[:specialty]
  end

  def self.all
    rows = DB.execute('SELECT * FROM doctors');
  end

  def is_new?
    @id.nil?
  end

  def new?
    is_new?
  end

  def has_been_saved?
    !is_new?
  end

  def save
    query = "INSERT INTO doctors (name, age, specialty) VALUES ('#{@name}', #{@age}, '#{@specialty}')"
    DB.execute(query);
    @id = DB.last_insert_row_id
  end
end

quinn = Doctor.new(name: 'Harley Quinn', age: 35)
p quinn
p quinn.is_new?
quinn.save
p quinn
p quinn.is_new?
quinn.age = quinn.age + 1;