def get_related_item_element(fk_element):
    """
    Returns related element by its foreign key
    
    It takes <fk /> element and finds related <item />
    element by attributes
    """
    fk_item_element_selector = '//model[@model="{}"]//item[@key="{}"]'.format(
        fk_element.attrib['model'], fk_element.attrib['key'])
    fk_item_element = fk_element.xpath(fk_item_element_selector)[0]
    return fk_item_element
