app.rb

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'
  gem 'rails', github: 'rails/rails'
  gem 'arel', github: 'rails/arel'
  gem 'rack', github: 'rack/rack'
  gem 'sprockets', github: 'rails/sprockets'
  gem 'sprockets-rails', github: 'rails/sprockets-rails'
  gem 'sass-rails', github: 'rails/sass-rails'
  gem 'sqlite3'
end

require 'active_record'
require 'minitest/autorun'
require 'logger'

# This connection will do for database-independent bug reports.
ActiveRecord::Base.establish_connection(adapter: 'sqlite3', database: ':memory:')
ActiveRecord::Base.logger = Logger.new(STDOUT)

ActiveRecord::Schema.define do
  create_table :properties, force: true  do |t|
     t.decimal :number # Replace this with t.integer and it works
  end
end

class Property < ActiveRecord::Base
  attr_accessor :number_should_be_validated_as_integer


  validates :number, numericality: {
    # Using a proc here is not needed to expose the problem, but is used as an idea why I can not
    # use integer as storage.
    only_integer: ->(item) { item.number_should_be_validated_as_integer }
  }#, if: ->() { number_changed? } # Enabling this line  works around the problem but feels like a hack
end

class BugTest < Minitest::Test
  def test_numericality_validation
    property = Property.new
    property.number_should_be_validated_as_integer = true
    property.number = '150'
    property.save! # Works like a charm

    property.reload

    # Nothing changed, same validations run, on the same input, but suddenly this starts to  fail.
    property.number_should_be_validated_as_integer = true
    assert property.save, "Should save. errors: #{property.errors.messages}" # Does not work
  end
end