spec.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(false) do
  source 'https://rubygems.org'

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

  gem 'activerecord', '6.1.3.1'
  gem 'sqlite3'
  gem 'rspec'
end

require 'active_record'
require 'rspec'

ActiveRecord::Base.establish_connection(adapter: 'sqlite3', database: ':memory:')
ActiveRecord::Base.logger = Logger.new(STDOUT)

ActiveRecord::Schema.define do
  create_table :posts do |t|
    t.string :title
  end
end

Post = Class.new(ActiveRecord::Base)

RSpec.describe do
  it '#previous_changes' do
    post = Post.create!(title: 'foo')
    post.update!(title: 'bar')

    expect(post.previous_changes).to eq('title' => ['foo', 'bar'])
  end
end

RSpec::Core::Runner.invoke