validations2.rb

require 'active_model' 
class User
  include ActiveModel::Validations
  attr_accessor :password
  validate do
  errors.add(:base, "Don't let dad choose the password.") if password == '1234'
  end 
end


user = User.new 
user.password = '12345' 
user.valid? # => true
user.password = '1234' 
user.valid? # => false