When I add to cart the product that triggers adding a free promotional product, the error occurs.

In case when you add to cart the product that  triggers adding a free promotional item, the following error occures:

SQLSTATE[23000]: Integrity constraint violation: 1452 Cannot add or

update a child row: a foreign key constraint fails

(`zadmin_winetalk`.`sales_flat_quote_item_option`, CONSTRAINT

`FK_5F20E478CA64B6891EA8A9D6C2735739` FOREIGN KEY (`item_id`)

REFERENCES `sales_flat_quote_item` (`item_id`) ON DELETE CASCADE ON U)

there is a conflict with a 3-d party extension that works with shopping carts.

Solution: 

Data Base request:

ALTER TABLE `sales_flat_quote_item_option` DROP FOREIGN KEY `FK_5F20E478CA64B6891EA8A9D6C2735739`