named_scope gets applied to self-referential relations

When performing a find using a named scope on an object which utilizes acts_as_tree, the scope is getting applied to the acts_as_tree relations in addition to the primary find request.

Example:

class Job < ActiveRecord::Base
  acts_as_tree

  named_scope :bases, :conditions => { :parent_id => nil }
end

Performing:

Job.bases.destroy_all

Will only destroy the base jobs and not their Job#children jobs. Here is the log output:

Job Load (1.5ms)   SELECT * FROM "jobs" WHERE ("jobs"."parent_id" IS NULL) 
SQL (0.2ms)   BEGIN
Job Load (7.6ms)   SELECT * FROM "jobs" WHERE ("jobs".parent_id = 1) AND ("jobs"."parent_id" IS NULL)
Job Destroy (0.2ms)   DELETE FROM "jobs" WHERE "id" = 1

Note that the automatically generated query of children is getting the bases scope added to it which will return zero records since a job's parent_id cannot be equal to 1 AND not null.

Had I directly queried the jobs WITHOUT a named scope, the automatic destruction of the children would happen as expected.