When is a Mock not a Mock

A couple of weeks ago I had to add a feature to some code that was written almost two years ago. It was a pretty simple web service client that constructed some XML, using Ruby’s REXML library, and made the web service call via another library.

A quick inventory shows two dependencies, the web service call and REXML, so that’s what I expected to see when I came across the tests. But what did I see? Paraphrasing:

1 @client_under_test = Client.new
2 @client_under_test.stub!(:send_message)
3 @client_under_test.stub!(:parse_response)

Of course the send_message call contained the construction of the XML so that there was no test that the XML being constructed was correct.

Heck that was all that was being done in the class, and a closer inspection of the tests revealed that all they were doing was testing the error handling by changing the return value of parse_response.

This is a classic sign that the developer at the time couldn’t figure out how to stub out the actual dependency, and so they did the next best thing, they stubbed out the caller.

I knew this was wrong, but I had an iteration meeting the next day and still had to update the .NET web service that this code was calling. So I started to just go ahead and make the untested change, but then my Clean Code wristband started burning until I couldn’t take it any more.

I had to get the REXML library under test. So what to do? Well the REXML construction looked like this:

1 doc = REXML::Document.new(create_schema)
2 doc.elements["name1"].text = "thing 1"
3 doc.elements["name2"].text = "thing 2"
4 ...

Okay I know how to do this normally. The first test was simple:

1 REXML::Document.should_receive(:new).with(schema)

It passed! What was so hard about testing this anyway? Okay let’s test that the elements are set:

1 ....
2 elements = mock(REXML::Document, :[] => mock_element)
3 doc = mock(REXML::Document, :elements => elements)
4 REXML::Document.stub!(:new).with(schema).and_return(doc)
5 ...

Oh that sucks! Since the API doesn’t run entirely through the doc object I have to stub elements, then stub each individual element, then should_receive on text, and if I want to be perfect I can stub out the array accessor too.

I have to should_receive for each element name, the tests will be hard to follow and fragile. Now I get why the original developer punted on this. There’s got to be a better way. I can start expecting actual XML, but from past experience I know that leads to very hard to read tests that give errors that aren’t easy to diagnose.

What I really care about is that the elements in this hash-like structure have their text element set to the right data. After trying out several approaches I had a moment of inspiration, I want a struct! Must be my inner C programmer.

If elements was a hash of structs, I could just check their text values are set. This way I’m testing that I’m using the REXML library right rather than testing the library itself. If only Ruby had a struct concept…

I suppose you’ve already figured out that it does, haven’t you?

1 MockElement = Struct.new("MockElement", :text)

What that does is setup a class named MockElement, with the accessor text. When is a Mock not a Mock? When it’s really a Fake.

Instead of stubbing out elements to return yet another mock, elements will return a hash of the element names I know I’m using that point to these fake elements. Then after making the call in the client, I’ll just check that the text is set properly. This is best demonstrated with an example:

 1 before(:each) do
 2   @elements = initial_elements_hash
 3   @doc = mock(REXML::Document, :elements => @elements)
 4   REXML::Document.stub!(:new).with(schema).and_return(@doc)
 5 end
 6 
 7 def initial_elements_hash
 8    return {"element1" => MockElement.new, 
 9           "element2" => MockElement.new}
10 end
11 
12 it "should set the data in the xml doc" do
13   @client.send_message
14 
15   @doc.elements["element1"].text.should == "Eric"
16   @doc.elements["element2"].text.should == "Smith"
17 end

Aha! Now I’ve tested that the XML was properly constructed without relying on looking at the actual XML text itself and without stubbing out the class itself. The code is cleaner, the tests are easier to understand, and I can implement my new feature. More importantly, my wristband stopped burning.

Eric Smith, Software Craftsman

Eric Smith has a Master's Degree in Video Game Development from DePaul University.