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This is more convenient to set a FK to null when it's unset, rather than
having to write 2 different insert statements

It only does binding a nullptr to sqlite, since we can't write to a
nullptr, nor can we assign nullptr to int/string/float/...

This will avoid random crash due to dangling pointer in case the DB was
released

So we don't have to call .c_str() every 10 lines

Objects were always created and added to cache, regardless of their
status in DB