How I Made Sure You Can't Like and Dislike the Same Post at Once

When building a reactions feature in Go with SQLite, a simple UNIQUE constraint on (user_id, target_type, target_id) prevents users from simultaneously liking and disliking the same post. The application logic then becomes a clean three-way switch: insert on first reaction, delete on same-button toggle, or update when switching between like and dislike. A test walking through all three state transitions confirms the counts never overlap.