PT 4 -> PT 5+ serialized attributes problem

test.rb

# frozen_string_literal: true

require 'securerandom'

$test_db_name = "test_#{SecureRandom.rand(10_000)}.db"
puts "Using #{$test_db_name} database"

# Let's make sure it definitely doesn't exist
File.delete $test_db_name if File.exist? $test_db_name

def setup_test(paper_trail_version:)
  puts "\n\n**************************"
  puts "Testing paper trail v#{paper_trail_version}"
  puts "**************************\n\n"

  # Use this template to report PaperTrail bugs.
  # Please include only the minimum code necessary to reproduce your issue.
  require "bundler/inline"

  # STEP ONE: What versions are you using?
  gemfile(true) do
    ruby "2.4.2"
    source "https://rubygems.org"
    gem "activerecord", "4.2.10"
    gem "minitest", "5.10.3"
    gem "paper_trail", paper_trail_version, require: false
    gem "sqlite3"
  end

  require "active_record"
  require "minitest/autorun"
  require "logger"

  # Please use sqlite for your bug reports, if possible.
  ActiveRecord::Base.establish_connection(adapter: "sqlite3", database: $test_db_name)
  yield if block_given?

  require "paper_trail/config"

  # STEP THREE: Configure PaperTrail as you would in your initializer
  PaperTrail::Config.instance.track_associations = true

  require "paper_trail"
end

fork do
  setup_test paper_trail_version: '4.0.0' do
    ActiveRecord::Base.logger = nil
    ActiveRecord::Schema.define do
      # STEP TWO: Define your tables here.
      create_table :users, force: true do |t|
        t.text :props
        t.timestamps null: false
      end

      create_table :versions do |t|
        t.string :item_type, null: false
        t.integer :item_id, null: false
        t.string :event, null: false
        t.string :whodunnit
        t.text :object, limit: 1_073_741_823
        t.text :object_changes, limit: 1_073_741_823
        t.integer :transaction_id
        t.datetime :created_at
      end
      add_index :versions, %i[item_type item_id]
      add_index :versions, [:transaction_id]

      create_table :version_associations do |t|
        t.integer  :version_id
        t.string   :foreign_key_name, null: false
        t.integer  :foreign_key_id
      end
      add_index :version_associations, [:version_id]
      add_index :version_associations, %i[foreign_key_name foreign_key_id],
                name: "index_version_associations_on_foreign_key"
    end
    ActiveRecord::Base.logger = Logger.new(STDOUT)
  end

  # STEP FOUR: Define your AR models here.
  class User < ActiveRecord::Base
    has_paper_trail
    serialize :props, Hash
  end

  # STEP FIVE: Please write a test that demonstrates your issue.
  class BugTest < ActiveSupport::TestCase
    def test_1
      # Create a version record (containing a serialized column) with PT 4
      user = User.create props: { foo: 'bar' }
      user.update props: { bar: 'baz' }

      assert_equal({ 'foo' => 'bar' }, user.previous_version.props)
    end
  end
end

Process.wait

fork do
  setup_test paper_trail_version: '5.0.0'

  # STEP FOUR: Define your AR models here.
  class User < ActiveRecord::Base
    has_paper_trail
    serialize :props, Hash
  end

  # STEP FIVE: Please write a test that demonstrates your issue.
  class BugTest < ActiveSupport::TestCase
    def test_1
      # Load a version record (containing a serialized column) with PT 5 (created with PT 4)
      assert_raises(ActiveRecord::SerializationTypeMismatch) { User.last.previous_version }
    end
  end
end

Process.wait

# Let's clean up after ourselves
File.delete $test_db_name