Fix: false positive of validate_presence_of

[HeitorMC]

Referent to #1302

When your model has validate_presence_of with a custom message:

class Article < ApplicationRecord
  validates_presence_of :owner, message: 'this is wrong, it needs the owner'
end

it shouldn't pass, but it does

    it { should_not validate_presence_of(:owner) }

However, this spec spec/unit/shoulda/matchers/active_model/validate_presence_of_matcher_spec.rb is failing in lines 554, 605, and 694.

I was checking with pry-rails and I saw that the matcher is passing through:

shoulda-matchers/lib/shoulda/matchers/active_model/allow_value_matcher.rb

Line 405 in 195153c

def failure_message

But it should pass through:

shoulda-matchers/lib/shoulda/matchers/active_model/allow_value_matcher.rb

Line 432 in 195153c

def failure_message_when_negated # rubocop:disable Metrics/MethodLength

Unfortunately, I have not found out why. @vsppedro @mcmire do you have any idea what might be going on?

[HeitorMC]

Hey, @mcmire @vsppedro any updates on this case?

[mcmire]

I have a long response that I tried to write here, but the short answer is that it's intentionally flipped. When used in the positive, the validate_presence_of matcher needs to test that the model does not allow nil; and when used in the negative, it needs to test that it does allow nil. Under the hood, the validate_presence_of matcher uses the allow_value matcher to do this, so this:

it { should_not validate_presence_of(:owner) }

is sort of like saying this:

it { should allow_value(nil).for(:owner) }

That's why it passes through failure_message and not failure_message_when_negated.