Similar to ActiveRecord Mongoid should support the "before_add", "after_add", "before_remove", "after_remove" callbacks
Here is the ActiveRecord documentation: http://api.rubyonrails.org/classes/ActiveRecord/Associations/ClassMethods.html
There isn't even really a workaround currently as before_save doesn't call on the parent due to the atomic push.
A previous issue
MONGOID-463 mentions this same issue and references a mysterious pull request that included this functionality.