Add missing file

server/core/mixins.py

@@ -0,0 +1,20 @@
+import graphene
+
+
+class HiddenForMixin:
+    hidden_for = graphene.List('users.schema.SchoolClassNode')
+
+    @staticmethod
+    def resolve_hidden_for(parent, info, **kwargs):
+        return parent.hidden_for.all()
+
+
+class VisibleForMixin:
+    visible_for = graphene.List('users.schema.SchoolClassNode')
+
+    @staticmethod
+    def resolve_visible_for(parent, info, **kwargs):
+        return parent.visible_for.all()
+
+class HiddenAndVisibleForMixin(HiddenForMixin, VisibleForMixin):
+    pass