Implement collection updating.

django_etesync/serializers.py

@@ -197,3 +197,19 @@ class CollectionSerializer(serializers.ModelSerializer):
                                     ).save()
 
         return instance
+
+    def update(self, instance, validated_data):
+        """Function that's called when this serializer is meant to update an item"""
+        revision_data = validated_data.pop('content')
+
+        with transaction.atomic():
+            main_item = instance.mainItem
+            # We don't have to use select_for_update here because the unique constraint on current guards against
+            # the race condition. But it's a good idea because it'll lock and wait rather than fail.
+            current_revision = main_item.revisions.filter(current=True).select_for_update().first()
+            current_revision.current = None
+            current_revision.save()
+
+            process_revisions_for_item(main_item, revision_data)
+
+        return instance