Add Expression::exprNow() support for all DB platforms

[DarkSide666]

No description provided.

[mvorisek]

I have looked into this - there is no functional test - please add one into SelectTest.

I would like to have both the SQL query/string and the result (matching format) asserted for every database.

Preferably with at least 6/microseconds precision.

The format should be unified for all DB vendors and parsable using regular DBAL datetime type.

[mvorisek]

In SelectTest there should be no Model/DBAL (typecasting) usage.

Test here somethings like "SELECT $e->exprNow()". You can you $this->debug = true; for debugging the sent queries.

Once you get results/formats you want, assert the sent query, see JSON testing for example, you will have to expect different strings for some engines.

Once you have this raw querying working, only then bother with Model/DBAL, otherwise you will have too many layers to debug at once.

[DarkSide666]

Looks like DBAL itself is quite broken with DATETIME and microseconds. Some engines partly support that, migrator mostly don't support and ... well, it's mostly DBAL which strips .u from datetime.

[mvorisek]

The DBAL limitation is tracked in #1313. Easy to fix in DBAL, but quite hard to fix in atk4/data. I have not enough time to support DBAL workarounds for everything...

This PR is hovewer perfect itself and once the tests are green I will merge it. Thank you for all the great ideas ❤, I basically only reviewed them ;-)

[mvorisek]

Thank you, great contibution!

[mvorisek]

@DarkSide666 one question, what is your exact usecase?

[DarkSide666]

I've hit it in one small internal app my colleague is developing and I was reviewing.
I guess 90% of time I use it to insert timestamp in DB without using php DateTime() and passing actual time to DB. Also for model default value for fields like created_at or in model hooks when you have to store precise time of event for example.

Thank you for your help! We both spent a lot of time on this :)