find_missing_indexes.rb

c = ActiveRecord::Base.connection
c.tables.collect do |t|  
  columns = c.columns(t).collect(&:name).select {|x| x.ends_with?("_id" || x.ends_with("_type"))}
  indexed_columns = c.indexes(t).collect(&:columns).flatten.uniq
  unindexed = columns - indexed_columns
  unless unindexed.empty?
    puts "#{t}: #{unindexed.join(", ")}"
  end
end