Allow for custom instantiation of input serializer

docs/rest-framework.rst

@@ -19,3 +19,29 @@ You can create a Mutation based on a serializer by using the
         class Meta:
             serializer_class = MySerializer
 
+
+Customizing the mutation
+--------
+
+By default, when the serializer is instantiated, only the input argument
+is passed. Sometimes, the Django `request` object is required in the serializer
+context. In fact, any sort of complicated serializer will probably require something
+like `request.user`. This can be performed by customizing the instantiation
+method as such:
+
+.. code:: python
+
+    from graphene_django.rest_framework.mutation import SerializerMutation
+
+    class MySecondAwesomeMutation(SerializerMutation):
+        
+        @classmethod
+        def instantiate_serializer(cls, instance, args, request, info):
+            
+            input = args.get('input')
+            
+            return cls._meta.serializer_class(data=dict(input),
+                                              context={'request': request})
+      
+        class Meta:
+            serializer_class = MySerializer

graphene_django/rest_framework/mutation.py

@@ -107,10 +107,16 @@ class SerializerMutation(six.with_metaclass(SerializerMutationMeta, Mutation)):
     )
     
     @classmethod
-    def mutate(cls, instance, args, request, info):
+    def instantiate_serializer(cls, instance, args, request, info):
+        
         input = args.get('input')
         
-        serializer = cls._meta.serializer_class(data=dict(input))
+        return cls._meta.serializer_class(data=dict(input))
+
+    @classmethod
+    def mutate(cls, instance, args, request, info):
+
+        serializer = cls.instantiate_serializer(instance, args, request, info)
 
         if serializer.is_valid():
             return cls.perform_mutate(serializer, info)