A site reported "internal server error" adding a second business unit. It was
reproducible: create one with a blank code, create another with a blank code,
500.
A column that is unique and nullable accepts any number of NULLs - that is what
makes "optional but unique" work - and exactly ONE empty string. The form sent
'', so the first blank code saved and every one after it collided with it. The
field showed no asterisk because it genuinely is optional; the database just
behaved as though it were not.
This is not specific to business units. A dozen columns across core and the
plugins are unique and nullable - asset numbers, hostnames, item codes, subnet
names, gage-lab tags - and each was one blank form away from the same 500.
Fixing them an endpoint at a time would have left the next to be found by a
user, so a before_flush listener normalises blank to NULL on any unique nullable
text column. Listening on Session rather than on individual mappers covers
plugin models imported later, and avoids mapper-event semantics that differ
between SQLAlchemy versions.
A genuine duplicate is now a 409 with a readable message rather than a bare 500
with a traceback in the log: reusing a code that is taken is the caller's
mistake, not a server fault.
Verified against the development database: three business units with blank codes
all save, the blank stores as NULL, and a real duplicate code returns 409.
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