[9.x] Fix default pivot attributes (#40947)

* [9.x] Fix default pivot attributes

* Add test

Author: iamgergo

src/Illuminate/Database/Eloquent/Relations/Concerns/AsPivot.php

@@ -76,7 +76,9 @@ public static function fromRawAttributes(Model $parent, $attributes, $table, $ex
 
         $instance->timestamps = $instance->hasTimestampAttributes($attributes);
 
-        $instance->setRawAttributes($attributes, $exists);
+        $instance->setRawAttributes(
+            array_merge($instance->getRawOriginal(), $attributes), $exists
+        );
 
         return $instance;
     }

tests/Integration/Database/EloquentCustomPivotCastTest.php

@@ -137,6 +137,17 @@ public function testCastsAreRespectedOnSyncArrayWhileUpdatingExisting()
         $this->assertEquals(['foo1' => 'bar1'], $project->collaborators[0]->pivot->permissions);
         $this->assertEquals(['baz2' => 'bar2'], $project->collaborators[1]->pivot->permissions);
     }
+
+    public function testDefaultAttributesAreRespectedAndCastsAreRespected()
+    {
+        $project = CustomPivotCastTestProject::forceCreate([
+            'name' => 'Test Project',
+        ]);
+
+        $pivot = $project->collaborators()->newPivot();
+
+        $this->assertEquals(['permissions' => ['create', 'update']], $pivot->toArray());
+    }
 }
 
 class CustomPivotCastTestUser extends Model
@@ -160,6 +171,10 @@ public function collaborators()
 
 class CustomPivotCastTestCollaborator extends Pivot
 {
+    protected $attributes = [
+        'permissions' => '["create", "update"]',
+    ];
+
     protected $casts = [
         'permissions' => 'json',
     ];