class project_issue(crm.crm_case, osv.osv):
    _columns = {'project_id': fields.many2one('project.project', 'Project'),
        'type_id': fields.many2one('project.task.type', 'Stages', domain=
        "[('project_ids', '=', project_id)]")}
How can I change the choices in an OpenERP selection field based on other field values?