Extended FlexMock Example Using Google4R

Google4R is a simple Ruby wrapper around the Google APIs. In this extended example, we will use FlexMock to test software that uses the Google APIs, without every communicating with Google itself.

Purchase.rb

Here is the bit of code that we will be testing…

  require 'google4r/checkout'
  require 'item'

  class Purchase

    def initialize(config)
      @frontend = Frontend.new(config)
      @frontend.tax_table_factory = TaxTableFactory.new
    end

    # Purchase the +quantity+ items identified by +item_id+.  Return the
    # confirmation page URL.
    def purchase(item_id, quantity=1)
      item = Item.find(item_id)
      checkout = @frontend.create_checkout_command
      checkout.cart.create_item do |cart_item|
        cart_item.name = item.name
        cart_item.description = item.description
        cart_item.unit_price = item.unit_price
        cart_item.quantity = quantity
      end
      response = checkout.send_to_google_checkout
      response.redirect_url
    end

  end

FrontEnd is a Google4R class that provides a lot of the front end work for talking to the Google APIs. The config object given to the Purchase initializer is simply a hash of values defining the merchant_id, merchant_key and sandbox flag. To use the real Google checkout APIs, you will need to obtains a merchant id and key from Google. Since we will be mocking the Google interaction, we can use dummy values in our test.

The tax table factory is required by the Google4R software. We provide the following simplified one. Read the Google API documents for more information.

  class TestTaxTableFactory
    def effective_tax_tables_at(time)
      tax_free_table = TaxTable.new(false)
      tax_free_table.name = "default table"
      tax_free_table.create_rule do |rule|
        rule.area = UsCountryArea.new(UsCountryArea::ALL)
        rule.rate = 0.0
      end
      return [tax_free_table]
    end
  end

Item is simply an ActiveRecord class that we are using to hold our purchase item information. It should respond to the name, description and unit_price messages.

Testing Without Using External Resources

Our first test attempt will be to run the purchase method without talking to either the live Google web services, or hitting an actual ActiveRecord database.

Mocking Active Record

The ActiveRecord part is easy to mock. The following will handle it:

  flexmock(Item).should_receive(:find).with(1).and_return(
    flexmock("guitar",
      :name => "Deschutes",
      :description => "Deschutes model Guitar",
      :unit_price => Money.new(2400.00)))

We have mocked out the find method on Item so that whenever we call find with an integer argument of 1, we will return a mock item that will report its name, description and unit_price. This gives us an item for testing without actually reading the database.

Mocking the Google Web Services Call

Next we want to prevent the Google4R API from actually talking to the live web service. Everything that happens in the purchase method is all done locally except for the final call to send_to_google_checkout. All we need to do is mock out that one method.

  flexmock(Google4R::Checkout::CheckoutCommand).new_instances do |instance|
    instance.should_receive(:send_to_google_checkout).once.
      and_return(flexmock(:redirect_url => "http://google.response.url"))
  end

When we ask FrontEnd to create a check out command, it returns an instance of Google4R::Checkout::CheckoutCommand. We then use flexmock to specify that when Google4R::Checkout::CheckoutCommand creates a new instance, it should actually return a partial mock of that instance. The block given to the new_instances method allows us to configure the mocked checkout command. We tell it return a response object (yes, another mock) that report our dummy response URL.

The Final Result

Here is the complete unit test:

  def test_buying_a_guitar
    # Setup
    flexmock(Item).should_receive(:find).with(1).and_return(
      flexmock("guitar",
        :name => "Deschutes",
        :description => "Deschutes model Guitar",
        :unit_price => Money.new(2400.00)))

    flexmock(Google4R::Checkout::CheckoutCommand).new_instances do |instance|
      instance.should_receive(:send_to_google_checkout).once.
        and_return(flexmock(:redirect_url => "http://google.response.url"))
    end

    # Execute
    p = Purchase.new({
      :merchant_id => 'dummy_id',
      :merchant_key => 'dummy_key',
      :use_sandbox => true })
    url = p.purchase(1)

    # Assert
    assert_equal "http://google.response.url", url
  end

Testing the Details

The above test is fine as far as it goes. It demonstrates how to use mocks to avoid talking to external resources such as databases and web services. But as a unit test, it is sorely lacking in several areas.

All the test really demonstrates is that the send_to_google_checkout method is called. There are no tests to ensure that the right item descriptions and prices are correctly stored in the cart. In fact, if we rewrote the purchase method as follows:

  def purchase(item_id, quantity=1)
    @frontend.create_checkout_command.send_to_google_checkout.redirect_url
  end

it would still pass the unit test we designed, even though the rewrite is obviously an incorrect implementation.

A more complete test is a bit more complicated. Here are the details.

Mocking Active Record

Our incorrect version of purchase never calls the find method of Item. We can easily test for that by adding a once constraint one that mock specification. Since find is a read-only method, we don‘t really care if it is called multiple times, as long as it is called at least one time, so we will add an at_least modifier as well.

Finally, we are going to break the guitar mock out into its own declaration. The reason will become obvious in a bit.

  mock_guitar = flexmock("guitar",
    :name => "Deschutes",
    :description => "Deschutes model guitar",
    :unit_price => Money.new(2400.00))

  flexmock(Item).should_receive(:find).with(1).at_least.once.
    and_return(mock_guitar)

Mocking a Cart Item

The next bit is a wee bit complicated, but we will handle it a little bit at a time so that it doesn‘t become overwhelming.

There are three main objects in the Google checkout API that we deal with in the next section.: (1) the checkout command object returned by the front end, (2) the cart object returned by the checkout command, and (3) the item passed to the block in the create_item call.

We will tackle them in reverse order, starting with the item objects given to the create_item block. The item must respond to four attribute assignments. This is straightforward to mock, just make sure you include the once constraint so that the assignments are required.

  mock_item = flexmock("item")
  mock_item.should_receive(:name=).with(mock_guitar.name).once
  mock_item.should_receive(:description=).with(mock_guitar.description).once
  mock_item.should_receive(:unit_price=).with(mock_guitar.unit_price).once
  mock_item.should_receive(:quantity=).with(1).once

Notice how we used the mock_guitar object defined earlier to provide values in the with constraint. This way we don‘t have to repeat the explicit strings and values we are checking. (Keep it DRY!).

Mocking the Cart

The mock cart object will pass the mock_item to a block when the create_item method is called. We specify that with the following:

  mock_cart = flexmock("cart")
  mock_cart.should_receive(:create_item).with(Proc).once.and_return { |block|
    block.call(mock_item)
  }

FlexMock objects can handle blocks passed to them by treating them as the final object in the calling list. Use Proc in the with constraint to match the block and then invoke the block explicitly via block.call(...) in the and_return specification.

Mocking the Checkout Command

Finally, we tie it all together by mocking the checkout command. As before, we use new_instances to force newly created checkout commands to be stubbed. This time we not only mockout the send_to_google method, but we also mock the cart command to return the carefully crafted mock_cart object from the previous section.

  flexmock(Google4R::Checkout::CheckoutCommand).new_instances do |instance|
    instance.should_receive(:cart).with().once.and_return(mock_cart)
    instance.should_receive(:send_to_google_checkout).once.
      and_return(flexmock(:redirect_url => "http://google.response.url"))
  end

The Final Test Method

Here is the complete detailed version of the test method.

  def test_buying_a_guitar_with_details
    # Setup
    mock_guitar = flexmock("guitar",
      :name => "Deschutes",
      :description => "Deschutes model guitar",
      :unit_price => Money.new(2400.00))

    flexmock(Item).should_receive(:find).with(1).at_least.once.
      and_return(mock_guitar)

    mock_item = flexmock("item")
    mock_item.should_receive(:name=).with(mock_guitar.name).once
    mock_item.should_receive(:description=).with(mock_guitar.description).once
    mock_item.should_receive(:unit_price=).with(mock_guitar.unit_price).once
    mock_item.should_receive(:quantity=).with(1).once

    mock_cart = flexmock("cart")
    mock_cart.should_receive(:create_item).with(Proc).once.and_return { |block|
      block.call(mock_item)
    }

    flexmock(Google4R::Checkout::CheckoutCommand).new_instances do |instance|
      instance.should_receive(:cart).with().once.and_return(mock_cart)
      instance.should_receive(:send_to_google_checkout).once.
        and_return(flexmock(:redirect_url => "http://google.response.url"))
    end

    # Execute
    p = Purchase.new({
      :merchant_id => 'dummy_id',
      :merchant_key => 'dummy_key',
      :use_sandbox => true })
    url = p.purchase(1)

    # Assert
    assert_equal "http://google.response.url", url
  end

Summary

Testing with mock objects can get complex. We used seven different mock or partial mock objects in testing the interaction of our code with the Google checkout API. Most testing scenarios won‘t require that many, but anytime your code touches something external, it might require a mock object for testing.

We should stop and ask ourselves: was it worth it? It seems like an awful lot of work just to test a very simple purchase method. Wouldn‘t it just be easier to just use the Google API directly for testing and forget about the mocks?

Perhaps, but using mock objects have several definite advantages:

  • You can run the test at any time without worrying whether Google, the internet, or anything else is up and connected.
  • You can easy test for error conditions using mock objects. For example, does your code correctly handle the case where you get an exception when connecting to google? Mocks can easily create those error conditions that are difficult to achieve with real objects.

    E.g.

       instance.should_receive(:send_to_google_checkout).once.
         and_return { raise Google4R::Checkout::GoogleCheckoutError }
    

Some might point out that in the final test method we are hardly using Google4R software at all, most of the code we interact with are mock objects. Doesn‘t that defeat the purpose of testing?

The answer is simple. Always keep in mind what you are testing. The goal of the TestPurchase test case is not the make sure the Google4R code is correct, but that our Purchase class correctly interoperates with it. We do that by carefully stating what methods are called with what arguments and what they return. The test just checks that we are using to external software as we expect it to. We don‘t actually care about the Google4R software itself in this test case (presumably we do have tests that cover Google4R, but those are different tests).

In the end, mock objects are a power tool to have in your testing toolbox.