modify _writable_fields in serializers.py

Sometimes 'self' instance is not a field (used in rest_framework)
If the 'serializer' that contains 'field' is 'self', only the subelements of
the serializer, the 'fields', are returned.
in the case of a serializer that contains 'field' as a 'child' element,
Only instances of `.source` ending with` _set '(following relationships
"backward") are found using regular expressions.
Then use the child elements again to find the field using '_writable_fields'.

Do not modify '_readable_fields' because it does not address the issue of what
is currently being viewed.

rest_framework/serializers.py

@@ -362,10 +362,15 @@ class Serializer(BaseSerializer):
 
     @cached_property
     def _writable_fields(self):
-        return [
-            field for field in self.fields.values()
-            if (not field.read_only) or (field.default is not empty)
-        ]
+        ret = []
+        for field in self.fields.values():
+            if re.compile("\_set$").findall(field.source):
+                for field in field.child._writable_fields:
+                    ret.append(field)
+                continue
+            if (not field.read_only) or (field.default is not empty):
+                ret.append(field)
+        return ret
 
     @cached_property
     def _readable_fields(self):