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Instead, have the caller specify which type they want.
This allows us avoid using those ugly std::static_pointer_cast

This is lighter, and allows us to use the interface type instead of
the implementation type as the key() method parameter.

This makes it easier to scan a row and extract values to variables,
without having to manually pass the column index.
In the likely event we add/remove columns, this will help prevent
regressions as well.

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