如何为只验证两个属性之一的模型编写规范（xor）？

Question

Is this a correct way to write a Model, Factory and Specs for a model with two alternative validations (neither both of have value set nor, both of them could be nil?

I did it the following way which works right. However, am not sure that this could the most elegant way of doing it.

models/invoice.rb

class Invoice < ActiveRecord::Base
  validates :payment_term, presence: true, allow_nil: true
  validates :interest_on_arrears, numericality: true, allow_nil: true
  validate :choose_xor_date
  private
  def choose_xor_date
    unless deadline.blank? ^ payment_term.blank?
      errors.add(:base, 'specify a deadline or a payment term. 
        Not both empty, nor both filled')
    end
  end
end

models/invoice_spec.rb

RSpec.describe Invoice, type: :model do
  describe 'validations' do
    it 'fails validation with both deadline and payment_term filled' do 
      invoice_with_deadline = build(:invoice, deadline: '2016-02-20', payment_term: '')
      invoice_with_payment_term = build(:invoice, deadline: '', payment_term: '2')
      invoice_with_deadline_and_payment_term =
        build(:invoice, deadline: '2016-02-20', payment_term: '2')
      expect(invoice_with_deadline).to be_valid
      expect(invoice_with_payment_term).to be_valid
      expect(invoice_with_deadline_and_payment_term).to be_invalid
    end 
  end
end

factories/invoice.rb

FactoryGirl.define do
  factory :invoice do
    deadline "2016-02-20"
    payment_term "2"
  end
end

features/invoice_feature_spec.rb

# User create with parameter when creating invoice local object. 
describe 'when user has invoice' do
  @invoice = create(:invoice, deadline: '2016-02-20', payment_term: '') 
  visit invoices_path
  click_link I18n.t('button.show')
end

Answer 1

You're almost there. Here's what I'd do differently:

In the model, only a couple of comments on naming:

• It's "in arrears", not "on arrears".
• I think validate_deadline_xor_payment_term or validate_deadline_or_payment_term_chosen would be better names for the validation method. "Choice" is actually a good English translation of XOR.

In the model spec,

• The single spec that you already have should be three specs, one object and expectation in each.
• You need a fourth spec to test that the model is invalid if neither field is set.
• You correctly set all the optional values explicitly, so it's easy to see what they are. If you changed your factory as I suggest below you could remove the blank ones.

In the factory, I would remove the default values. If a value is optional, the factory with the unqualified name shouldn't have the optional value. It's easy for the reader to remember that convention and it makes sense that an unspecified value is nil. If a factory with an unqualified name does have an optional value set, there is no way for the reader to know what the value is.

The feature spec

• should be in an it block, not a describe block. Actually, since it's a feature spec , it would read even better to surround the entire feature in a feature block and use scenario for this example.
• needs expectations about what is on the index page ( expect(page).to have_content and the like)
• needs expectations about what is on the show page