From 60290e5c3440629afbed571799bb07df3b8aceaa Mon Sep 17 00:00:00 2001
From: Yared Yemane <yaredyemane1@gmail.com>
Date: Wed, 29 Apr 2026 03:36:45 -0700
Subject: [PATCH] Swap module sort_order on conflict during update.

When updating a module sort_order to an occupied position in the same course, perform an atomic swap in a transaction instead of failing with a unique constraint error.

Made-with: Cursor
---
 internal/repository/lms_modules.go | 71 +++++++++++++++++++++++++++++-
 1 file changed, 70 insertions(+), 1 deletion(-)

diff --git a/internal/repository/lms_modules.go b/internal/repository/lms_modules.go
index 599d2d9..2a59390 100644
--- a/internal/repository/lms_modules.go
+++ b/internal/repository/lms_modules.go
@@ -93,13 +93,78 @@ func (s *Store) ListModulesByProgramAndCourse(ctx context.Context, programID, co
 }
 
 func (s *Store) UpdateModule(ctx context.Context, id int64, input domain.UpdateModuleInput) (domain.Module, error) {
+	q, tx, err := s.BeginTx(ctx)
+	if err != nil {
+		return domain.Module{}, err
+	}
+	defer tx.Rollback(ctx)
+
+	var current dbgen.Module
+	err = tx.QueryRow(ctx, `
+SELECT id, program_id, course_id, name, description, icon, sort_order, created_at, updated_at
+FROM modules
+WHERE id = $1
+FOR UPDATE
+`, id).Scan(
+		&current.ID,
+		&current.ProgramID,
+		&current.CourseID,
+		&current.Name,
+		&current.Description,
+		&current.Icon,
+		&current.SortOrder,
+		&current.CreatedAt,
+		&current.UpdatedAt,
+	)
+	if err != nil {
+		if errors.Is(err, pgx.ErrNoRows) {
+			return domain.Module{}, pgx.ErrNoRows
+		}
+		return domain.Module{}, err
+	}
+
+	if input.SortOrder != nil {
+		targetSort := int32(*input.SortOrder)
+		if targetSort != current.SortOrder {
+			var conflictID int64
+			err = tx.QueryRow(ctx, `
+SELECT id
+FROM modules
+WHERE course_id = $1
+  AND sort_order = $2
+  AND id <> $3
+FOR UPDATE
+`, current.CourseID, targetSort, id).Scan(&conflictID)
+			if err != nil && !errors.Is(err, pgx.ErrNoRows) {
+				return domain.Module{}, err
+			}
+			if err == nil {
+				var tempSort int32
+				if err := tx.QueryRow(ctx, `
+SELECT COALESCE(MIN(sort_order), 0) - 1
+FROM modules
+WHERE course_id = $1
+`, current.CourseID).Scan(&tempSort); err != nil {
+					return domain.Module{}, err
+				}
+
+				if _, err := tx.Exec(ctx, `UPDATE modules SET sort_order = $1 WHERE id = $2`, tempSort, id); err != nil {
+					return domain.Module{}, err
+				}
+				if _, err := tx.Exec(ctx, `UPDATE modules SET sort_order = $1 WHERE id = $2`, current.SortOrder, conflictID); err != nil {
+					return domain.Module{}, err
+				}
+			}
+		}
+	}
+
 	var nameText pgtype.Text
 	if input.Name != nil {
 		nameText = pgtype.Text{String: *input.Name, Valid: true}
 	} else {
 		nameText = pgtype.Text{Valid: false}
 	}
-	m, err := s.queries.UpdateModule(ctx, dbgen.UpdateModuleParams{
+	m, err := q.UpdateModule(ctx, dbgen.UpdateModuleParams{
 		ID:          id,
 		Name:        nameText,
 		Description: optionalTextUpdate(input.Description),
@@ -112,6 +177,10 @@ func (s *Store) UpdateModule(ctx context.Context, id int64, input domain.UpdateM
 		}
 		return domain.Module{}, err
 	}
+
+	if err := tx.Commit(ctx); err != nil {
+		return domain.Module{}, err
+	}
 	return moduleToDomain(m), nil
 }