user_validator.rb

class UserValidator
  include ActiveModel::Validations

  def self.fields
    [:username, :password, :email, :phone, :age]
  end

  attr_accessor(*fields)

  validates :username,
    length: { within: 5..20 },
    uniqueness: true,
    format: { with: /\A\w*\z/ }
 
  validates :password, length: { within: 5..20 }
  validates :email, format: { with: /.+@.+\..+/i }
  validates :phone, format: { with: /\+48/ }
  validates :age, numericality: true
end