Sometimes you just want the IDs of a set of objects instead of creating a bunch of model objects. It turns out this is easy to do.

For example, if we want to get back a list of tag ids in our Product model to do some processing/munging:

   1  
   2  def tag_ids
   3      sql = "SELECT tag_id FROM products_tags WHERE product_id = #{self.id} ORDER BY tag_id ASC"
   4      ActiveRecord::Base.connection.select_values(sql).collect! { |x| x.to_i }
   5  end

Voila, an array of integers. Much faster, depending on the size of your model object.