diff --git a/drf_writable_nested/mixins.py b/drf_writable_nested/mixins.py
index f40dd48..4c6678f 100644
--- a/drf_writable_nested/mixins.py
+++ b/drf_writable_nested/mixins.py
@@ -170,6 +170,8 @@ def update_or_create_reverse_relations(self, instance, reverse_relations):
                 )
             elif not related_field.many_to_many:
                 save_kwargs[related_field.name] = instance
+            elif related_field.many_to_many and related_field.remote_field.through:
+                save_kwargs['parent_instance'] = instance
 
             new_related_instances = []
             errors = []
@@ -177,6 +179,9 @@ def update_or_create_reverse_relations(self, instance, reverse_relations):
                 obj = instances.get(
                     self._get_related_pk(data, field.Meta.model)
                 )
+                if not obj and (getattr(field, 'only_assignation_allowed', False) or
+                                getattr(field.parent, 'only_assignation_allowed', False)):
+                    continue
                 serializer = self._get_serializer_for_field(
                     field,
                     instance=obj,
@@ -206,12 +211,16 @@ def update_or_create_direct_relations(self, attrs, relations):
         for field_name, (field, field_source) in relations.items():
             obj = None
             data = self.get_initial()[field_name]
+
             model_class = field.Meta.model
             pk = self._get_related_pk(data, model_class)
             if pk:
                 obj = model_class.objects.filter(
                     pk=pk,
                 ).first()
+            else:
+                if getattr(field, 'only_assignation_allowed', False):
+                    continue
             serializer = self._get_serializer_for_field(
                 field,
                 instance=obj,