The column parent_id on object hr.department must be set as ondelete='cascade' or 'restrict'

Note: This only happens with hr_department_sequence, error does not appear with hr.

The parent_id field in the hr.department object does not have an on_delete value which causes OpenERP to display the following message when loading records which specify parents through an xml:

ERROR db openerp.osv.orm: The column parent_id on object hr.department must be set as ondelete='cascade' or 'restrict'

Steps to reproduce (Module included as tar.bz2):
 - Create a module which depends on hr_department_sequence
 - To the module, add an hr_data.xml file such as this:

<?xml version = "1.0" encoding="utf-8"?>
<openerp>
  <data noupdate="1">

    <record id="dep_parent" model="hr.department">
      <field name="name">Parent department</field>
    </record>

    <record id="dep_child" model="hr.department">
      <field name="name">Child department</field>
      <field name="parent_id" ref="dep_parent"/>
    </record>

</data>
</openerp>

 - Start OpenERP, installing the module, the message will display

This is not a show stopper: OpenERP will keep running, but it can trigger testing environments.