I want to write a class A that has a method calculate(<params>). That method should calculate a value using database data. So I wrote a class Test_A for unit testing (TDD). The database access is done using another class which I have mocked with a class, let's call it Accessor_Mockup.

Now, the TDD cycle requires me to add a test that fails and make the simplest changes to A so that the test passes. So I add data to Accessor_Mockup and call A.calculate with appropriate parameters.

But why should A use the accessor class at all? It would be simpler (!) if the class just "knows" the values it could retrieve from the database. For every test I write I could introduce such a new value (or an if-branch or whatever).

But wait ... TDD is more. There is the refactoring part. But that sounds to me like "OK, I can do this all with a big if-elseif construct. I could refactor it using a new class ... but instead I make use of the DB accessor and do this in a totally different way. The code will not necessarily look better afterwards but I know I WANT to use the database".