feat(backend/db): add database error handler

backend/db/db.go

@@ -2,11 +2,13 @@ package db
 
 import (
 	"context"
+	"errors"
 	"fmt"
 	"net/http"
 	"time"
 
 	"github.com/jackc/pgx/v5"
+	"github.com/jackc/pgx/v5/pgconn"
 	"github.com/jackc/pgx/v5/pgxpool"
 )
 
@@ -48,3 +50,29 @@ func transaction(handler func(context.Context, pgx.Tx) (statusCode int, err erro
 	}
 	return
 }
+
+// Handle database error
+func handleDBError(errIn error) (statusCode int, err error) {
+	if errIn == nil {
+		return
+	}
+	if errors.Is(errIn, pgx.ErrNoRows) {
+		err = fmt.Errorf("not found")
+		statusCode = http.StatusNotFound
+		return
+	}
+	var pgErr *pgconn.PgError
+	if errors.As(errIn, &pgErr) {
+		switch pgErr.Code {
+		case "22P02", "22007": // Invalid data format
+			err = fmt.Errorf("%s", pgErr.Message)
+			statusCode = http.StatusBadRequest
+			return
+		case "23505": // Unique constraint violation
+			err = fmt.Errorf("already exists")
+			statusCode = http.StatusConflict
+			return
+		}
+	}
+	return http.StatusInternalServerError, errIn
+}