How to get new values each test

[AndreLeBlanc]

I am writing tests where data is put into a database with unique constraints. customerEmail has to be a unique value for example. Is there a way to write a test so that if I do 100 runs I won't get the same value?

Some JSONs I am generating have up to 10 unique constraints, and with a hundred test-runs I am regularly getting test failures.

[gruhn]

Sorry, if I'm misunderstanding but does your test look somewhat like this?

fc.assert(
  fc.property(
    fc.emailAddress(),
    customerEmail => {
      // ...
      database.insert({ customerEmail })
      // ...
    }
  )
)

Because the strong assumption with these tests is that no state persists across test runs. Otherwise this can badly mess with shrinking. For example, let's say the test fails (for whatever reason) when there is a customer email in the database that contains a "+". One test run inserts [email protected] provoking the issue. Now the shrinker tries to find a simpler counterexample but it can suddenly reproduce the issue for any input email address, so in the end you get the counterexample [email protected] which is too simple and doesn't actually reproduce the issue in isolation.

The test above checks "what happens if I insert a single record into the database" in a many different ways. But it sounds like what you really want to test is "what happens if I insert many records into the database". For that I would write the test like this:

fc.assert(
  fc.property(
    fc.uniqueArray(fc.emailAddress()),
    customerEmails => {
      database.reset() 
      // ...
      for (const customerEmail of customerEmails) {
        database.insert({ customerEmail })
      }
      // ...
    }
  )
)

Now I can also use fc.uniqueArray to make sure I get pairwise different email addresses. Just make sure to reset the database as good as possible after or before each test run, so no state is carried over.