[nrr]

I think it's probably worth mentioning that the principal concern for tests should be proving out the application's logic, and unless you're really leaning on your database to be, e.g., a source of type and invariant enforcement for your data, any sort of database-specific testing can be deferred to integration and UAT.

I use both the mocked and real database approaches illustrated here because they ultimately focus on different things: the mocked approach validates that the model is internally consistent with itself, and the real database approach validates that the same model is externally consistent with the real world.

It may seem like a duplication of effort to do that, but tests are where you really should Write Everything Twice in a world where it's expected that you Don't Repeat Yourself.

[jcgrillo]

The database is often the thing that enforces the most critical application invariants, and is the primary source of errors when those invariants are violated. For example, "tenant IDs are unique" or "updates to the foobars are strictly serializable". The only thing enforcing these invariants in production is the interplay between your database schema and the queries you execute against it. So unless you exercise these invariants and the error cases against the actual database (or a lightweight containerized version thereof) in your test suite, it's your users who are actually testing the critical invariants.

I'm pretty sure "don't repeat yourself" thinking has led to the vast majority of the bad ideas I've seen so far in my career. It's a truly crippling brainworm, and I wish computer schools wouldn't teach it.

[nrr]

> The only thing enforcing these invariants in production is the interplay between your database schema and the queries you execute against it."

I'm unsure that I agree. The two examples you gave, establishing that IDs are unique and that updates to entities in the system are serializable (and linearizable while we're here), are plenty doable without having to touch the real database. (In fact, as far as the former is concerned, this dual approach to testing is what made me adopt having a wholly separate "service"[0] in my applications for doling out IDs to things. I used to work in a big Kafka shop that you've almost certainly heard of, and they taught me how to deal with the latter.)

That said, I'd never advocate for just relying on one approach over the other. Do both. Absolutely do both.

> I'm pretty sure "don't repeat yourself" thinking has led to the vast majority of the bad ideas I've seen so far in my career. It's a truly crippling brainworm, and I wish computer schools wouldn't teach it.

I brought up WET mostly to comment that, if there's one place in software development where copying and pasting is to be encouraged, testing is it. I'd like to shelve the WET vs. DRY debate as firmly out of scope for this thread if that's alright.

0: It's a service inasmuch as an instance of a class implementing an interface can be a service, but it opens up the possibility of more easily refactoring to cross over into running against multiple databases later.

[marcosdumay]

There's something very, very wrong in the way we write programs nowadays.

Because yeah, the database is you main source of invariants. But there is no good reason for you application environment not to query the invariants from there and test or prove your code around them.

We do DRY very badly, and the most vocal proponents are the worst... But I don't think this is a good example of the principle failing.