rails/rails — pull request #7889 by chatura-atapattu (2012-10-09 20:35:54+00:00)
Title: Add 'mixed_options_for_select' helper to generate combined bare option and optgroup tags from nested array

mixed_options_for_select([["Pop"], ["Rock", [["Alternative Rock"], ["Hard Rock"]]], ["Country"]])

Pop

Alternative Rock

Hard Rock

Country

Currently not possible.