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class User < ApplicationRecord
  validates :login, :email, presence: true

  before_validation :ensure_login_has_a_value

  private
    def ensure_login_has_a_value
      if login.nil?
        self.login = email unless email.blank?
      end
    end
end

class User < ApplicationRecord
  validates :login, :email, presence: true

  before_create do
    self.name = login.capitalize if name.blank?
  end
end

class User < ApplicationRecord
  before_validation :normalize_name, on: :create

  # :on takes an array as well
  after_validation :set_location, on: [ :create, :update ]

  private
    def normalize_name
      self.name = name.downcase.titleize
    end

    def set_location
      self.location = LocationService.query(self)
    end
end

class User < ApplicationRecord
  after_initialize do |user|
    puts "You have initialized an object!"
  end

  after_find do |user|
    puts "You have found an object!"
  end
end

irb> User.new
You have initialized an object!
=> #<User id: nil>

irb> User.first
You have found an object!
You have initialized an object!
=> #<User id: 1>

class User < ApplicationRecord
  after_touch do |user|
    puts "You have touched an object"
  end
end

irb> u = User.create(name: 'Kuldeep')
=> #<User id: 1, name: "Kuldeep", created_at: "2013-11-25 12:17:49", updated_at: "2013-11-25 12:17:49">

irb> u.touch
You have touched an object
=> true

class Employee < ApplicationRecord
  belongs_to :company, touch: true
  after_touch do
    puts 'An Employee was touched'
  end
end

class Company < ApplicationRecord
  has_many :employees
  after_touch :log_when_employees_or_company_touched

  private
    def log_when_employees_or_company_touched
      puts 'Employee/Company was touched'
    end
end

irb> @employee = Employee.last
=> #<Employee id: 1, company_id: 1, created_at: "2013-11-25 17:04:22", updated_at: "2013-11-25 17:05:05">

irb> @employee.touch # triggers @employee.company.touch
An Employee was touched
Employee/Company was touched
=> true

throw :abort

class User < ApplicationRecord
  has_many :articles, dependent: :destroy
end

class Article < ApplicationRecord
  after_destroy :log_destroy_action

  def log_destroy_action
    puts 'Article destroyed'
  end
end

irb> user = User.first
=> #<User id: 1>
irb> user.articles.create!
=> #<Article id: 1, user_id: 1>
irb> user.destroy
Article destroyed
=> #<User id: 1>

class Order < ApplicationRecord
  before_save :normalize_card_number, if: :paid_with_card?
end

class Order < ApplicationRecord
  before_save :normalize_card_number,
    if: Proc.new { |order| order.paid_with_card? }
end

class Order < ApplicationRecord
  before_save :normalize_card_number, if: Proc.new { paid_with_card? }
end

class Comment < ApplicationRecord
  before_save :filter_content,
    if: Proc.new { forum.parental_control? },
    unless: Proc.new { author.trusted? }
end

class Comment < ApplicationRecord
  before_save :filter_content,
    if: [:subject_to_parental_control?, :untrusted_author?]
end

class PictureFileCallbacks
  def after_destroy(picture_file)
    if File.exist?(picture_file.filepath)
      File.delete(picture_file.filepath)
    end
  end
end

class PictureFile < ApplicationRecord
  after_destroy PictureFileCallbacks.new
end

class PictureFileCallbacks
  def self.after_destroy(picture_file)
    if File.exist?(picture_file.filepath)
      File.delete(picture_file.filepath)
    end
  end
end

class PictureFile < ApplicationRecord
  after_destroy PictureFileCallbacks
end

PictureFile.transaction do
  picture_file_1.destroy
  picture_file_2.save!
end

class PictureFile < ApplicationRecord
  after_commit :delete_picture_file_from_disk, on: :destroy

  def delete_picture_file_from_disk
    if File.exist?(filepath)
      File.delete(filepath)
    end
  end
end

class PictureFile < ApplicationRecord
  after_destroy_commit :delete_picture_file_from_disk

  def delete_picture_file_from_disk
    if File.exist?(filepath)
      File.delete(filepath)
    end
  end
end

class User < ApplicationRecord
  after_create_commit :log_user_saved_to_db
  after_update_commit :log_user_saved_to_db

  private
  def log_user_saved_to_db
    puts 'User was saved to database'
  end
end

irb> @user = User.create # prints nothing

irb> @user.save # updating @user
User was saved to database

class User < ApplicationRecord
  after_save_commit :log_user_saved_to_db

  private
  def log_user_saved_to_db
    puts 'User was saved to database'
  end
end

irb> @user = User.create # creating a User
User was saved to database

irb> @user.save # updating @user
User was saved to database