Rails 3, SQL for Association - enabling a union, Arel?

In the class below I have a method which returns the value of two
associations concatenated (#configured_resources), e.g. the union is
done via ruby instead of via SQL. The issue is that I don't get a
proxy back which messes up a subsequent call
#to_xml(include: :my_concat_association). So it seems the better way
is issue a SQL query with a UNION but I'd rather not write out the
association SQL by hand--seems more logical to have a method something
like #association_union( :assoc1, :assoc2) which would read the
association SQL from the two associations I have defined and produce
the appropriate union statement. Trouble is I can't figure out how to
get the SQL out of the association. I've looked at the
AssociationCollection and other proxies, but it isn't adding up for

Any pointer would be helpful.

class ApplicationInstance < ActiveRecord::Base

  belongs_to :packaged_application

  # http://guides.rubyonrails.org/association_basics.html#the-has_many-association
  # http://my.safaribooksonline.com/book/web-development/ruby/9780132480345/advanced-active-record/ch09lev1sec7

  has_many :application_resources

  has_many :schema_resource_sets, :through => :application_resources,
           :source => :configurable, :source_type =>

  has_many :example_resources, :through => :application_resources,
           :source => :configurable, :source_type => 'ExampleResource'

  def configured_resources
    # schema_resource_sets:
    # SELECT "schema_resource_sets".*
    # FROM "schema_resource_sets"
    # INNER JOIN "application_resources"
    # ON "application_resources".configurable_id =
    # AND "application_resources".configurable_type =
    # WHERE (("application_resources".application_instance_id = 1))

    schema_resource_sets + example_resources