http://random-rails.blogspot.com/2010/08/searhcing-over-multiple-columns.html

operating_system.rb

class OperatingSystem < ActiveRecord::Base
  belongs_to :host

  def self.search(term)
    t = OperatingSystem.arel_table

    match_on = [:name,:version,:service_pack]

    matches = match_on.map {|m| "IF(#{t[m].matches("%#{term}%").to_sql},operating_systems.#{m.to_s},NULL)" }
    matching_string = "CONCAT_WS(' ', #{matches.join(",")}) as matched_on"
 
    results = OperatingSystem.select(
      "*",
      matching_string
    ).having("matched_on != ''").joins(:host)

    results.map do |os|
      host = os.host
      host.matched_on ||= []
      host.matched_on << os.matched_on
      host
    end
  end
end