diff --git a/backend/app/Schedule/EloquentScheduleRepository.php b/backend/app/Schedule/EloquentScheduleRepository.php
index 8cbde2a..214e8a5 100644
--- a/backend/app/Schedule/EloquentScheduleRepository.php
+++ b/backend/app/Schedule/EloquentScheduleRepository.php
@@ -122,16 +122,6 @@ class EloquentScheduleRepository implements ScheduleRepository
     public function updateAssignment(
         ScheduleAssignment $assignment,
     ): ScheduleAssignment {
-        $query = ScheduleAssignmentModel::query()
-            ->whereKey($assignment->getId());
-        if ($assignment->getCompletedAt() === null) {
-            $query->update(['completed_at' => null]);
-        } else {
-            $query->whereNull('completed_at')->update([
-                'completed_at' => $assignment->getCompletedAt(),
-            ]);
-        }
-
         $model = ScheduleAssignmentModel::find($assignment->getId());
         if ($model === null) {
             throw new DomainException(
@@ -139,6 +129,9 @@ class EloquentScheduleRepository implements ScheduleRepository
             );
         }
 
+        $model->completed_at = $assignment->getCompletedAt();
+        $model->save();
+
         return $this->assignmentToDomain($model);
     }