While I think this is a good idea for smaller data sets, it just isn't feasible for larger data sets that require indices. Having every combination of columns as indices is just hell for the RDBMS. When you are faceting with a large dataset, it's better to use something like ElasticSearch or Sphinx.
Nice. I've built something very similar in my current app. In my app it's called `SearchModel` and descendants live within the `app/models` dir. I include some modules from `ActiveModel` to make it act as a regular db-backed model, which allows me to use it with `form_for`.