numericality validator blank string bug (https://github.com/rails/rails/issues/31870)

numericality_blank.rb

# frozen_string_literal: true

# rubocop:disable Documentation

begin
  require 'bundler/inline'
rescue LoadError => e
  $stderr.puts 'Bundler version 1.10 or later is required. Please update your\
    Bundler'
  raise e
end

gemfile(true) do
  source 'https://rubygems.org'

  git_source(:github) { |repo| "https://github.com/#{repo}.git" }

  # Activate the gem you are reporting the issue against.
  gem 'sqlite3'
  # gem 'rails', '~> 4.1.0' # works as expected
  gem 'rails', '~> 4.2.0' # fails
  # gem 'rails', '~> 5.1.0' # fails and MiniTest throws an error
  # gem 'rails', '~> 5.2.0.rc1' # fails and MiniTest throws an error
end

require 'rack/test'
require 'action_controller/railtie'
require 'active_record'

# This connection will do for database-independent bug reports.
ActiveRecord::Base.establish_connection adapter: 'sqlite3',
                                        database: ':memory:'

ActiveRecord::Schema.define do
  create_table :student_classes do |t|
    t.integer :girls_count
    t.integer :boys_count
    t.integer :total_count
    t.boolean :discriminates_gender
  end
end

class StudentClass < ActiveRecord::Base
  validates :total_count,
            presence: {
              unless: :discriminates_gender?
            },
            numericality: {
              only_integer: true,
              allow_blank: true
            }
  validates :girls_count,
            presence: {
              if: :discriminates_gender?
            },
            numericality: {
              only_integer: true,
              allow_blank: true
            }
  validates :boys_count,
            presence: {
              if: :discriminates_gender?
            },
            numericality: {
              only_integer: true,
              allow_blank: true
            }

  def total_count
    sum = [girls_count, boys_count].compact.sum

    self[:total_count] || sum
  end
end

require 'minitest/autorun'

# Ensure backward compatibility with Minitest 4
Minitest::Test = MiniTest::Unit::TestCase unless defined?(Minitest::Test)

class BugTest < Minitest::Test
  def assert_valid(model, msg = nil)
    assert model.valid?, [msg, model.errors].compact.to_json
  end

  def setup
    @mixed = StudentClass.new discriminates_gender: false
    @gendered = StudentClass.new discriminates_gender: true
  end

  def test_total_can_be_nil
    @gendered.total_count = nil
    @gendered.girls_count = 5
    @gendered.boys_count = 5

    assert_valid @gendered
  end

  def test_total_can_be_blank
    @gendered.total_count = ''
    @gendered.girls_count = '5'
    @gendered.boys_count = '5'

    assert_valid @gendered
  end

  def test_genders_can_be_nil
    @mixed.total_count = 10
    @mixed.boys_count = nil
    @mixed.girls_count = nil

    assert_valid @mixed
  end

  def test_genders_can_be_blank
    @mixed.total_count = '10'
    @mixed.boys_count = ''
    @mixed.girls_count = ''

    assert_valid @mixed
  end
end