Does this work with `manager.model = instance.__class__` ? It'd probably be better. 

Hell maybe we should be paranoid and use something like `manager.model = instance._meta.proxy_for_model or instance.__class__`. If you have an `Addon` child class, like `Webapp`, you might want to compare against all `Addon`s, since they share the same database table. Webapps and Addons don't use the same field for slugs, and AFAIK Webapp is the only class that inherit from Addon, so this should never be a problem, but just in case...