diff --git a/rest_framework/serializers.py b/rest_framework/serializers.py
index 02c24b70e..8e7206fd8 100644
--- a/rest_framework/serializers.py
+++ b/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):