You can't accept a write if you're going to break a foreign key constraint when you insert the row. And I struggle to imagine a case where you want to do complex validation like "multiple of x" or "matches this regex" but don't want to actually check if the thing exists.
I used matching regexes to check if a particular string actually has a time moment, in the form YYYY-MM-DD hh:mm:ss.xxx .
If you add events, they may not have FKs, you just add them to the table while generating PKs on the fly. This is useful in many cases. There could be other approaches with different cases, I suspect.
