Current project I'm working on tries to do index work as background jobs. Where index worker method is like this:

  def perform(model)
    callback = ThinkingSphinx::RealTime.callback_for(model.class.name.underscore)
    callback&.after_commit model
  end

This mostly works but not for models using STI (single-table inheritance). For models where index is defined on the base_class, then one has to use model.class.base_class and for models where index is on descendant, then #base_class shouldn't be used. So it is an error prone and ugly business.

Would be useful to be able to add callbacks like:

ThinkingSphinx::Callbacks.append(self, {behaviours: [:real_time], background: true})

Or some other way to allow that easily.

It also applies to ts:index. It would be useful to queue indexing in background job batches. Presently we use the above indexation worker and can't benefit from the new clean-up functionality.

== thinking-sphinx 5.4 on rails 5 (in process of upgrading).

/related to #1038/