ActiveModel::Model - show nested errors

gistfile1.rb

class PersonData
  include ActiveModel::Model
  include Virtus.model

  attribute :pesel, String
  attribute :id_type, String

  validates :pesel, presence: true
  validates :id_type, presence: true
end

class CompanyData
  include ActiveModel::Model
  include Virtus.model

  attribute :name, String
  attribute :nip, String

  validates :name, presence: true
  validates :nip, presence: true
end

class Partner
  include ActiveModel::Model
  include Virtus.model

  attribute :test
  attribute :person_data, PersonData
  attribute :company_data, CompanyData
  # decyduje, czy company_data będzie walidowane
  attribute :check_company, Boolean, default: false

  validates :test, presence: true

  # zawsze uruchamiane przy valid?
  # na podstawie pierwszego komentarza z http://apidock.com/rails/ActiveRecord/Validations/ClassMethods/validates_associated
  validate do
    [:person_data].each do |attr|
      errors.add(attr, :invalid) if send(attr) && send(attr).invalid?
    end
  end

  # warunkowa walidacja
  # ":if - Specifies a method, proc or string to call to determine
  #  if the validation should occur (e.g. if: :allow_validation,
  #  or if: Proc.new { |user| user.signup_step > 2 }).
  #  The method, proc or string should return or evaluate to a true or false value."
  validate :validate_company, if: :check_company

  private

  def validate_company
    errors.add(:company_data, :invalid) if company_data.nil? || company_data.invalid?
  end
end

p = Partner.new(person_data: {pesel: 123})
p.valid?
# => false

p.errors.full_messages
# => ["Test nie może być puste", "Person data jest nieprawidłowe"]

p = Partner.new(check_company: true, company_data: {nip: -1})
p.valid?
# => false

p.errors.full_messages
# => ["Test nie może być puste", "Company data jest nieprawidłowe"]

if p.errors.any?
  p.errors.full_messages.each do |message|
    puts message
  end
end

# ->

if p.errors.any?
  p.errors.each do |attribute, message|
    puts p.errors.full_message(attribute, message)
  end
end

# ->

if p.errors.any?
  p.errors.each do |attribute, message|
    if p.send(attribute).respond_to?(:errors)
      p.send(attribute).errors.full_messages.each do |m|
        puts "#{attribute} (#{p.errors.full_message(attribute, '').strip}) - #{m}"
      end
    else
      puts p.errors.full_message(attribute, message)
    end
  end
end