pass instance to view.get_serializer in metadata's determine_actions method since choices (especially querysets of related fields) can often depend on the instance

rest_framework/metadata.py

@@ -74,19 +74,23 @@ class SimpleMetadata(BaseMetadata):
         actions = {}
         for method in set(['PUT', 'POST']) & set(view.allowed_methods):
             view.request = clone_request(request, method)
+            instance = None
             try:
                 # Test global permissions
                 if hasattr(view, 'check_permissions'):
                     view.check_permissions(view.request)
                 # Test object permissions
                 if method == 'PUT' and hasattr(view, 'get_object'):
-                    view.get_object()
+                    instance = view.get_object()
             except (exceptions.APIException, PermissionDenied, Http404):
                 pass
             else:
                 # If user has appropriate permissions for the view, include
                 # appropriate metadata about the fields that should be supplied.
-                serializer = view.get_serializer()
+                kwargs = {}
+                if instance is not None:
+                    kwargs['instance'] = instance
+                serializer = view.get_serializer(**kwargs)
                 actions[method] = self.get_serializer_info(serializer)
             finally:
                 view.request = request