diff --git a/Application.php b/Application.php
index 982cbfa..e7a61b3 100644
--- a/Application.php
+++ b/Application.php
@@ -63,7 +63,8 @@ public static function isGuest()
     public function login(UserModel $user)
     {
         $this->user = $user;
-        $primaryKey = $user->primaryKey();
+        $className = get_class($user);
+        $primaryKey = $className::primaryKey();
         $value = $user->{$primaryKey};
         Application::$app->session->set('user', $value);
 
diff --git a/db/DbModel.php b/db/DbModel.php
index d65a42c..cca989b 100644
--- a/db/DbModel.php
+++ b/db/DbModel.php
@@ -20,7 +20,7 @@ abstract class DbModel extends Model
 {
     abstract public static function tableName(): string;
 
-    public function primaryKey(): string
+    public static function primaryKey(): string
     {
         return 'id';
     }