meet it

A declares its validations, then checks itself.

A model is a class for one kind of thing your app stores, such as a user or an order. Instead of checking data by hand everywhere, a model declares each , a rule its data must follow, with a class macro. Rails writes validates :email, presence: true. Ours takes a message and a block.

The macro only stores the rule. errors runs each stored rule on the object with instance_eval, uses reject to drop the rules that pass, and keeps the messages of the rest. is then errors.empty?, the question Rails asks before every save.

class Signup < Struct.new(:email, :age, keyword_init: true)
  RULES = []
  def self.validates(message, &rule)
    RULES << [message, rule]
  end
  validates("email needs an @") { email.include?("@") }
  validates("age must be 13 or over") { age >= 13 }
  def errors
    RULES.reject { |message, rule| instance_eval(&rule) }.map(&:first)
  end
end
signup = Signup.new(email: "sam.example.com", age: 15)
p signup.errors
p Signup.new(email: "ana@example.com", age: 15).errors.empty?

class Signup < Struct.new(...) inherits named fields from a struct, so the model needs no initialize of its own.

guess first

A pet with a blank name and twelve legs. What will this show?

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

class Pet < Struct.new(:name, :legs, keyword_init: true)
  RULES = []
  def self.validates(message, &rule)
    RULES << [message, rule]
  end
  validates("name can't be blank") { !name.strip.empty? }
  validates("legs must be 0 to 8") { legs.between?(0, 8) }
  def errors
    RULES.reject { |message, rule| instance_eval(&rule) }.map(&:first)
  end
end
p Pet.new(name: " ", legs: 12).errors

your turn

Your turn. Add a validation for product names.

Write a message for the name rule in the first blank, then choose a minimum name length of 2 or 3. A mug should pass every rule.

Your code

press run to see what happens

fix it

A perfectly good email crashes the check. Fix the rule.

Broken code to fix

press run to see what happens

The error says include? was called on nil. Why is @email empty?
A struct keeps its fields itself, not in instance variables, so @email is nil. Use the reader.
On line 6, change @email to email, then press run.

nailed it

your model checks itself now. :D

A model declares its validations with a class macro.
errors runs every rule and keeps the failures' messages.
valid? is true only when errors is empty.

new words in your collection

model
validation
valid?

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

A model declares its validations, then checks itself.

A pet with a blank name and twelve legs. What will this show?

Your turn. Add a validation for product names.

A perfectly good email crashes the check. Fix the rule.

your model checks itself now. :D

A declares its validations, then checks itself.