It looks like there have been a couple of plugins developed in past years for Rails/ActiveScaffold to export an ActiveScaffold index view as CSV. One of them (last commits around 5 years ago) is referenced in the answer here:
How to Make CSV Format Report of User Information using Active Scaffold in Rails 2.3.8
The projects I can find look like they're dead, and despite searching I'm not finding updated versions.
Before I dive in and modernise one of these, I wanted to ask: is there a current (ActiveScaffold 3.4) plugin that will allow CSV export?
edit: found https://github.com/naaano/active_scaffold_export which was last updated in 2013
It turns out that, despite its 2-year old update status, Naaano's GitHub project took almost nothing to get working with ActiveScaffold 3.4.
Head over to https://github.com/naaano/active_scaffold_export, grab the code, and follow the very well written install/config instructions. Then simply modify the gemspec to require AS >=3.4.0, rebuild the gem, and you're good to go.