to_xml format with sti

Hello,
I use STI for some models and there seems to be a problem with xml served. here is my setup

class SalesDocument < ActiveRecord::Base
  has_many :items
end

class SalesQuotation < SalesDocument
end

class SalesOrder < SalesDocument
end

when all records are sales_quotations xml returned on /sales_documents.xml is the same as /sales_quotations.xml

or when all records are sales_orders xml returned on /sales_documents.xml is the same as /sales_orders.xml

<sales-orders type="array">
  <sales-order>
    <customer-id type="integer" nil="true"></customer-id>
    <delivery-date type="date">2009-07-24</delivery-date>
    <id type="integer">2053932786</id>
    <net-value type="float" nil="true"></net-value>
    <items type="array">
      <item>
        <id type="integer">2053932786</id>
        <line>10</line>
        <quantity type="float" nil="true"></quantity>
        <sales-document-id type="integer">2053932786</sales-document-id>
      </item>
    </items>
  </sales-order>
</sales-orders>

but when some of the records are sales_quotations and some are sales_records /sales_documents.xml returns

<records type="array">
  <record>
    <customer-id type="integer" nil="true"></customer-id>
    <delivery-date type="date">2009-07-24</delivery-date>
    <id type="integer">2053932786</id>
    <net-value type="float" nil="true"></net-value>
    <items type="array">
      <item>
        <id type="integer">2053932786</id>
        <line>10</line>
        <quantity type="float" nil="true"></quantity>
        <sales-document-id type="integer">2053932786</sales-document-id>
      </item>
    </items>
  </record>
  <record>
    <customer-id type="integer" nil="true"></customer-id>
    <delivery-date type="date">2009-07-24</delivery-date>
    <id type="integer">2053932787</id>
    <net-value type="float" nil="true"></net-value>
    <items type="array"/>
  </record>
</records>

I am assuming the correct format should be

<sales-documents type="array">
  <sales-order>
    ...
  </sales-order>
  <sales-quotation>
    ...
  </sales-quotation>
</sales-documents>

Interestingly /sales_documents.json returns correct set of elements

[

    *
      -
      {
          o
            -
            sales_order: {
                + delivery_date: "2009-07-24"
                + id: 2053932786
                + net_value: null
                + customer_id: null
                +
                  -
                  items: [
                      #
                        -
                        {
                            * quantity: null
                            * line: "10"
                            * sales_document_id: 2053932786
                            * id: 2053932786
                        }
                  ]
            }
      }
    *
      -
      {
          o
            -
            sales_quotation: {
                + delivery_date: "2009-07-24"
                + id: 2053932787
                + net_value: null
                + customer_id: null
                +
                  -
                  items: [
                  ]
            }
      }

]