extend rails3 activerecord public interface with powerfull connection.select_value(s) method, put this file to config/initializers/

active_record_select_attributes_extension.rb

module ActiveRecordSelectAttributesExtension
  def select_id(field = 'id')
    select_values(field)
  end

  def select_values(field = nil)
    scope = field ? self.scoped.select(field) : self.scoped
    connection.select_values(scope.to_sql)
  end

  def select_attributes(fields = nil)
    scope = fields ? self.scoped.select(fields) : self.scoped
    connection.send(:select_all, scope.to_sql)
  end
end

::ActiveRecord::Base.class_eval do
  extend ActiveRecordSelectAttributesExtension
end

Fascinatingly, in Rails 3.0.7, select_values fails for anything that has been scoped using 'where'. However, select_id continues to work, despite the fact that it just calls select_values.

This is because the class ActiveRecord::Relation actually defines select_values as a simple accessor with no arguments, so if you do something

User.where(:active => true).select_values(:id)

It fails with an ArgumentError: wrong number of arguments (1 for 0)

But if you call

User.where(:active => true).select_id

it proxies over to the User model, which then has select_values defined appropriately.

As written, you could actually use select_id for any field, but I think it is probably better to rename select_values to something like select_columns or something similar