Manual smoke test caught a bug: POST /persons with a duplicate name
returned 500 with the body 'insert person Cameron' instead of the
intended 409 Conflict.
Root cause: the handler keyed on `format!("{}", e).contains("unique")`,
but anyhow's plain Display only renders the *outermost* context
("insert person Cameron") and hides the diesel error nested below
('UNIQUE constraint failed: persons.name'). The string check was a
false negative on every duplicate.
Fix: walk the source chain and downcast for
diesel::result::Error::DatabaseError(UniqueViolation, _) — exposed
via a shared `is_unique_violation` helper used by both
create_person_handler and update_person_handler. Error bodies for
non-unique failures now use `{:#}` so the body actually carries the
underlying cause when the user surfaces it.
merge_persons_handler also moves to `{:#}` for richer error bodies;
the "itself" check was already structural and unaffected.
Regression test (faces::tests::is_unique_violation_walks_chain) pins
both the bug shape ({} doesn't surface UNIQUE) and the fix
(is_unique_violation correctly downcasts the chain), so a future
refactor of error handling can't silently re-bury this.
cargo test --lib: 171 / 0; fmt + clippy clean.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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