Unit test example for cache in model class

book.rb

class Book < ActiveRecord::Base
  after_commit :flush_cache

  def self.cached_find(id)
    Rails.cache.fetch([name, id], expires_in: 30.mitues) do
      where(:id => id).take!
    end
  end

  private

  def flush_cache
    Rails.cache.delete([self.class.name, id])
  end
end

book_test.rb

require "test_helper"

class BookTest < ActiveSupport::TestCase
  include CachingHelper

  def test_cached_find_raises_exception_if_missing_id_is_given
    assert_raise ActiveRecord::RecordNotFound do
      Book.cached_find(0)
    end
  end

  def test_cached_find_returns_cache_after_cache_was_created
    with_caching do
      book = books(:linux_book)
      assert_equal book, Book.cached_find(book.id)

      # mocha
      # see https://github.com/freerange/mocha
      # e.g.
      # Book.expects(:where).with(:id => book.id).times(0)
      # assert_equal book, Book.cached_find(book.id)

      # minitest/mock
      # see https://github.com/seattlerb/minitest/blob/master/lib/minitest/mock.rb
      mock = Minitest::Mock.new
      mock.expect(:take!, :not_called)
      Book.stub(:where, mock) do
        assert_equal book, Book.cached_find(book.id)
        assert_raise MockExpectationError do
          mock.verify
        end
      end
    end
  end

  def test_cached_find_loads_exact_record_after_cache_was_destroyed
    with_caching do
      book = books(:linux_book)
      assert_equal book, Book.cached_find(book.id)

      # This will invoke `after_commit` callback by test_after_commit gem
      # see https://github.com/grosser/test_after_commit
      assert book.update_attribute(:title, "New Linux Book")
      book.reload

      # mocha
      # e.g.
      # Book::ActiveRecord_Relation.any_instance
      #  .expects(:take!).returns(book).times(1)
      # Book.cached_find(book.id)
      # Rails.cache.clear

      # minitest/mock
      # see https://github.com/seattlerb/minitest/blob/master/lib/minitest/mock.rb
      mock = Minitest::Mock.new
      mock.expect(:take!, book)
      Book.stub(:where, mock) do
        assert_equal book, Book.cached_find(book.id)
        mock.verify
      end
    end
  end
end

books_controller.rb

class BooksController < ApplicationController
  def show
    @book = Book.cached_find(params[:id])
  end
end

caching_helper.rb

module CachingHelper
  def with_caching
    cache_store_type_origin = Rails.application.config.cache_store
    cache_store_origin = Rails.cache
    Rails.application.config.cache_store = [
      :memory_store, {:size => 64.megabytes }
    ]
    Rails.cache = ActiveSupport::Cache::MemoryStore.new(
      :expires_in => 1.minute
    )
    yield
  ensure
    Rails.cache.clear
    Rails.cache = cache_store_origin
    Rails.application.config.cache_store = cache_store_type_origin
  end
end