Check source attrs when checking for nested writes.

rest_framework/serializers.py

@@ -801,9 +801,8 @@ def raise_errors_on_nested_writes(method_name, serializer, validated_data):
     #     ...
     #     address = serializer.CharField('profile.address')
     assert not any(
-        '.' in field.source and
-        (key in validated_data) and
-        isinstance(validated_data[key], (list, dict))
+        len(field.source_attrs) > 1 and
+        (field.source_attrs[0] in validated_data)
         for key, field in serializer.fields.items()
     ), (
         'The `.{method_name}()` method does not support writable dotted-source '