Principal engineer code review

### Overall Assessment The function's intent is immediately obvious, which is a real strength — nobody will misread what it returns. Its two weaknesses are both structural rather than cosmetic: it issues one database round trip per user, and it raises an unhandled exception the first time an id has no matching row. ### Prioritized Recommendations 1. Replace the per-id query with a single `IN` query to eliminate the N+1 pattern. 2. Decide explicitly what should happen when a user id has no row. 3. Return early on an empty `user_ids` list so no query is issued at all. ### Detailed Feedback **[PERFORMANCE]** - N+1 database query **Original Code:** ```python for user_id in user_ids: user = db.query(User).filter(User.id == user_id).one() emails.append(user.email) ``` **Suggested Improvement:** ```python if not user_ids: return [] users = db.query(User).filter(User.id.in_(user_ids)).all() return [user.email for user in users] ``` **Rationale:** On a request path, a team of 200 people costs 200 round trips here; the rewrite costs one. If `user_ids` can grow without bound, chunk it — most engines cap the number of bind parameters allowed in an `IN` clause. **[CORRECTNESS]** - Unhandled `NoResultFound` **Rationale:** `.one()` raises `NoResultFound` when an id is missing, so a single stale id turns a partial result into a 500. Note that this is a behaviour change: the `in_()` rewrite silently drops missing ids instead of raising. If the caller needs to tell "no such user" apart from "user has no email", compare the returned count against `len(user_ids)` and log or raise deliberately rather than letting the difference disappear.