Refactor Store into SessionStore; move domain SQL onto User/Question/Answer.

internal/store/vote.go

@@ -0,0 +1,34 @@
+package store
+
+import (
+	"context"
+	"database/sql"
+	"fmt"
+)
+
+// Vote toggles or sets a user's vote on a question (value must be 1 or -1).
+func Vote(ctx context.Context, db *sql.DB, userID, questionID string, value int) error {
+	if value != 1 && value != -1 {
+		return fmt.Errorf("invalid vote")
+	}
+	tx, err := db.BeginTx(ctx, nil)
+	if err != nil {
+		return err
+	}
+	defer tx.Rollback()
+	var current sql.NullInt64
+	err = tx.QueryRowContext(ctx, `SELECT value FROM votes WHERE user_id = $1 AND question_id = $2`, userID, questionID).Scan(&current)
+	if err != nil && err != sql.ErrNoRows {
+		return err
+	}
+	if err == nil && current.Valid && int(current.Int64) == value {
+		_, err = tx.ExecContext(ctx, `DELETE FROM votes WHERE user_id = $1 AND question_id = $2`, userID, questionID)
+	} else {
+		_, err = tx.ExecContext(ctx, `INSERT INTO votes (user_id, question_id, value) VALUES ($1, $2, $3)
+ON CONFLICT (user_id, question_id) DO UPDATE SET value = excluded.value`, userID, questionID, value)
+	}
+	if err != nil {
+		return err
+	}
+	return tx.Commit()
+}