Sunspot: force index of parent model when updating child model

Question

I am using Sunspot to generate alot of my apps indexes and overviews.

In this app i have 2 models which have a parent/child one-to-many relationship. With Sunspot I index the number of childs a parent has, so this is available for sorting, scoping etc.

However, when I change the child model the parent model does not automatically get reindexed (as it hasn't changed). Forcing a parent.save through a call_back on the child doesn't force the index either.

So before I start hacking away:

What would be the best way to force an index action on the parent class in Sunspot when a child model gets changed/added?