The relevant bits of my model

class Lesson < ActiveRecord::Base
  has_many :listings
  has_many :courses, :through => :listings

  has_many :taggings
  has_many :tags, :through => :taggings

  if respond_to? :define_index
    define_index do
      indexes :name
      indexes description
      indexes tags(:name), :as => :tag_names
      indexes courses.name, :as => :course_name

      has taggings.tag_id, :as => :tag_ids
    end
  end
end

I've run a rebuild and restarted my server, but it keeps throwing this error

ThinkingSphinx::SphinxError: index lesson_core: query error: no field 'created_at' found in schema