1.rb represents a situation I have now... 2.rb proposes an addition to the ThinkingSphinx DSL for handling association attribute updates.

1.rb

# This example allows users to search through their memberships (to groups).
# The Membership index has a number of attributes that depend on both the
# membership itself and the group to which it belongs.

class User < ActiveRecord::Base
  has_many :memberships
  has_many :groups, :through => :memberships
end

class Membership < ActiveRecord::Base

  belongs_to :user
  belongs_to :group

  # Attributes:
  # * user_id
  # * group_id
  # * delta
  # * state ("pending", "active", "suspended")

  define_index do
    has user_id, group_id
    indexes group(:name)
    indexes group(:description)
    has "memberships.state = 'pending'", :as => :is_pending, :type => :boolean
    has "memberships.state = 'active'", :as => :is_active, :type => :boolean
    has "memberships.state = 'suspended'", :as => :is_suspended, :type => :boolean
    has "groups.state = 'pending'", :as => :is_group_pending, :type => :boolean
    has "groups.state = 'active'", :as => :is_group_active, :type => :boolean
    has "groups.state = 'archived'", :as => :is_group_archived, :type => :boolean
    # This would probably be a ts-delayed-delta (or ts-resque-delta), but for
    # the sake of this example let's make it synchronous.
    set_property :delta => true
  end
end

class Group < ActiveRecord::Base
  has_many :memberships
  has_many :users, :through => :memberships

  # Attributes:
  # * name
  # * description
  # * state ("pending", "active", "archived")
end

# Now, if a group's state is updated, it is necessary to update all the Sphinx
# attributes of its associated memberships. If a group's name or description
# are updated, it is necessary to update all associated memberships to have
# delta = 1 and then run the delta indexer for the memberships index.

2.rb

class Membership < ActiveRecord::Base
  # ...
  define_index do
    # ...
    has "(groups.state = 'active' AND groups.memberships_count > 1000)", :as => :is_big_group, :type => :boolean do
      # A class to watch (in the case of associations), default is watching
      # self.
      watch :group
      # A list of attributes which should be watched for "changed?" If none is
      # listed, updating a group will always update all the group's
      # memberships' attributes in searchd with the latest value of the
      # "value" block in an after_commit hook.
      attributes :state, :memberships_count
      # A ruby block equivalent to the SQL snippet to be evalutated in the
      # scope of the watched instance.
      value do
        self.state == "active" && self.memberships_count > 1000
      end
    end
    # ...
  end
end

# I wonder if there's a way to use arel to avoid the duplication of specifying
# the SQL snippet and the value block in ActiveRecord 3.0.x...