meet it

A runs your code automatically at a set moment.

People type emails with stray spaces and capitals. Rather than tidying them in every place that saves a user, the model declares a callback: code that runs automatically at a set moment. means: right before every save, run this.

Like validates, before_save is a class macro that only stores the block. save runs each stored callback with instance_eval, in the order they were declared, and only then stores the record. Rails also accepts a method name, as in before_save :tidy_email, and calls it with send.

class User < Struct.new(:email, keyword_init: true)
  CALLBACKS = []
  def self.before_save(&callback)
    CALLBACKS << callback
  end
  before_save { self.email = email.strip }
  before_save { self.email = email.downcase }
  def save
    CALLBACKS.each { |callback| instance_eval(&callback) }
    puts "Saved #{email}"
  end
end
User.new(email: "  Ana@Example.COM ").save

Inside a callback, write self.email =, because a bare email = would make a new local variable instead.

guess first

The slug is made before the title is tidied. What will this show?

Guess before you run it. Wrong guesses are how you learn, and nothing is marked.

class Post < Struct.new(:title, :slug, keyword_init: true)
  CALLBACKS = []
  def self.before_save(&callback)
    CALLBACKS << callback
  end
  before_save { self.slug = title.downcase.gsub(" ", "-") }
  before_save { self.title = title.strip }
  def save
    CALLBACKS.each { |callback| instance_eval(&callback) }
    p [title, slug]
  end
end
Post.new(title: " Hello World ").save

your turn

Your turn. Tidy a pet before it is saved.

In the first blank, use the macro that stores a callback. In the second, choose the text method that turns DOG into dog.

Your code

press run to see what happens

fix it

The tidy-up callback crashes. Fix it.

Broken code to fix

press run to see what happens

The error says strip was called on nil. Which email is on the right of the =?
email = ... makes a new local variable, which starts as nil. To call the setter, Ruby needs self..
Write self.email = email.strip.downcase inside the block on line 6, then press run.

nailed it

your model tidies itself before saving. :D

A callback runs your code at a set moment.
before_save stores callbacks, and save runs them in order.
Inside a callback, set fields with self..

new words in your collection

callback
before_save

This lesson isn’t saved. A free account remembers every lesson you finish.

A callback runs your code automatically at a set moment.

The slug is made before the title is tidied. What will this show?

Your turn. Tidy a pet before it is saved.

The tidy-up callback crashes. Fix it.

your model tidies itself before saving. :D

A runs your code automatically at a set moment.