missing FROM-clause

class InventoryLine(ModelSQL, ModelView):
    'Stock Inventory Line'
    __name__ = 'stock.inventory.line'

    product_code = fields.Function(
        fields.Char('Code',
            order_field='("product_product".code IS NULL) %(order)s, "product_product".code %(order)s'),
        'get_product_code',
        searcher='search_product_code')

    @classmethod
    def search_product_code(cls, name, clause):
        return [('product.code',) + tuple(clause[1:])]

    def get_product_code(self,name=''):
        return self.product.code