add notes on AR dup and rspec change

.rspec

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+--color
+--format d

AR_dup_and_rspec_change.rb

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+# frozen_string_literal: true
+
+require "bundler/inline"
+
+gemfile(true) do
+  source 'https://rubygems.org'
+
+  git_source(:github) { |repo| "https://github.com/#{repo}.git" }
+
+  gem 'activerecord', '5.2.3'
+  gem 'sqlite3', '1.4.1'
+  gem 'rspec', '3.8.0'
+  gem 'pry'
+end
+
+require 'active_record'
+require 'pry'
+
+ActiveRecord::Base.establish_connection(adapter: 'sqlite3', database: ':memory:')
+ActiveRecord::Migration.create_table(:users)
+ActiveRecord::Migration.create_table(:roles) do |t|
+  t.integer :user_id
+end
+
+class Role < ActiveRecord::Base
+  belongs_to :user
+end
+
+class User < ActiveRecord::Base
+  has_many :roles
+end
+
+# user = User.new
+# binding.pry
+# roles = user.roles.build
+# binding.pry
+# duplicated = user.roles.dup
+# binding.pry
+
+RSpec.describe 'change matcher' do
+  let(:user) { User.new }
+  let(:action) { ->{ user.roles.build } }
+
+  let(:assert_before) { ->{ be_empty } }
+  let(:assert_after) { ->{ contain_exactly(an_instance_of(Role)) } }
+
+  it 'demonstrates change works properly' do
+    sum = 0
+    expect{ sum += 1 }.to change { sum }.from(0).to(1)
+  end
+
+  it 'works when tested without change syntax' do
+    expect(user.roles).to(assert_before.call)
+    action.call
+    expect(user.roles).to(assert_after.call)
+  end
+
+  it 'works when tested with change syntax' do
+    expect{ action.call }
+      .to change{ user.roles }
+      .from(assert_before.call)
+      .to(assert_after.call)
+  end
+end
+
+RSpec::Core::Runner.invoke
+