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  • "you provide some test data where you exactly know what the outcome of the calculation should be" is fine for "pure" functions, but for data that change over time is impossible. I mean, today the Product #1 may have 2 resellers, tomorrow 5. So, if I make the expected result "static", today the test may pass and tomorrow it may fail. Commented May 13, 2019 at 7:06
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    @Bellons No, it won't. The resellers are part of the test data. Commented May 13, 2019 at 7:35
  • @Bellons: test data should be stored in test database, apart from production, with controlled changes to the data. And a well known principle for unit tests is to make sure each tests always works with a defined input. I thought this was so obvious that I did not see any necessity to mention it. Commented May 13, 2019 at 13:57
  • "a well known principle for unit tests is to make sure each tests always works with a defined input" is exactly the part I'm referring. I've always thought that was referred to functions like find the sum of two integers or find the minimal path in a graph. But data on DB are meant to change over time. The purpose of unit testing is to test small pieces of code, isn't it? So you are testing functionalities, not data (even if in a test environment). Commented May 13, 2019 at 14:27
  • A solution could be the usage of a throwaway db that creates data with expected properties in a "sandbox" environment. That way the stored could be tested with real input data and a real expected result, independent of the status of the whole system. Commented May 13, 2019 at 14:31