Fixed cart discount calculated incorrectly when product first added to cart.

[aligent-lturner]

Preconditions (*)

1. Magento 2.3.4
2. PHP 7.1+
3. Create a cart price rule for a fixed amount for the whole cart (e.g. $1). The rule should have no conditions attached to it (or restrictions on the items it applies to)
4. Create at least one configurable product with at least one simple product attached to each configurable.
5. Set the price of each item to be $10 (actual price doesn't matter - just used $10 for example)

Steps to reproduce (*)

1. Navigate to PDP of configurable product
2. Select options for the configurable and add to cart

Expected result (*)

1. Item is added to cart
2. Cart total is $9 ($10 for item - $1 discount)

Actual result (*)

1. Item is added to cart
2. Cart total is $9.50 ($10 for item - $0.50 discount)

Notes

The issue is also present any time a new simple product is added to the cart. For example, if a different configurable option was chosen, the discount would be calculated as $0.67 (then $0.75 for another). The reason for this behaviour is due to the following:

When a configurable is added to the cart, two items actually get added as Quote items - the configurable, and the simple item representing the selected options.
As part of the rule calculation, it skips child (simple) items by checking the result of getParentItemId():

magento2/app/code/Magento/SalesRule/Model/Validator.php

Lines 385 to 388 in e6e6725

	//Skipping child items to avoid double calculations
	if ($item->getParentItemId()) {
	continue;
	}

However, for a newly added item, parentItemId is not set yet, as this is only set in the beforeSave function of the quote item:

magento2/app/code/Magento/Quote/Model/Quote/Item/AbstractItem.php

Lines 178 to 185 in e6e6725

	public function beforeSave()
	{
	parent::beforeSave();
	if ($this->getParentItem()) {
	$this->setParentItemId($this->getParentItem()->getId());
	}
	return $this;
	}

Instead, the Validator class should be checking the result of getParentItem

[magento-deployment-service]

Thanks for opening this issue!

[m2-assistant]

Hi @aligent-lturner. Thank you for your report.
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• Steps to reproduce
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[aligent-lturner]

@magento give me 2.4-develop instance

[magento-engcom-team]

Hi @aligent-lturner. Thank you for your request. I'm working on Magento 2.4-develop instance for you

[magento-engcom-team]

Hi @aligent-lturner, here is your Magento instance.
Admin access: https://i-26622-2-4-develop.instances.magento-community.engineering/admin_9f98
Login: 277ecaa4 Password: ef4621a07619
Instance will be terminated in up to 3 hours.

[aligent-lturner]

Luma does not display discounts in minicart, and I do not have access to the underlying database to confirm the issue in the provided vanilla instance.

[aligent-lturner]

Actually - I was able to use the database backup functionality to confirm the issue in vanilla instance:

From quote table, after adding 2 $10 items:

INSERT INTO `quote` VALUES ('1',1,'2020-02-01 02:13:05','2020-02-01 02:17:19',NULL,1,0,0,'2','2.0000','0','0.0000','0.0000','USD','USD','USD','19.3300','19.3300',NULL,NULL,'3','0',NULL,NULL,NULL,NULL,NULL,NULL,NULL,NULL,1,0,'180.150.113.242','1',NULL,NULL,NULL,'USD','1.0000','1.0000',NULL,NULL,'20.0000','20.0000','19.3300','19.3300','1',0,NULL,NULL,0);

Note the values $19.33 (i.e. a discount of $0.67 has been applied, rather than $1)

And from the quote_item table:

INSERT INTO `quote_item` VALUES ('3','1','2020-02-01 02:15:23','2020-02-01 02:17:19','3',1,NULL,0,'Test_Product-Blue','Test Product',NULL,'1',NULL,0,0,'1.0000','1.0000','10.0000','10.0000',NULL,'0.0000','0.3300','0.3300','0.0000','0.0000','0.0000','10.0000','10.0000','0.0000','1.0000','configurable',NULL,NULL,NULL,NULL,NULL,'10.0000','10.0000','10.0000','10.0000','0.0000','0.0000',NULL,0,NULL,NULL,NULL,NULL,NULL,NULL,NULL,NULL,NULL),('4','1','2020-02-01 02:15:23','2020-02-01 02:15:23','1',1,'3',0,'Test_Product-Blue','Test Product-Blue',NULL,NULL,NULL,0,0,'1.0000','1.0000','10.0000','0.0000',NULL,'0.0000','0.0000','0.0000','0.0000','0.0000','0.0000','0.0000','0.0000','0.0000','0.0000','simple',NULL,NULL,NULL,NULL,NULL,NULL,NULL,NULL,NULL,NULL,NULL,NULL,0,NULL,NULL,NULL,NULL,NULL,NULL,NULL,NULL,NULL),('5','1','2020-02-01 02:17:19','2020-02-01 02:17:19','3',1,NULL,0,'Test_Product-Red','Test Product',NULL,'1',NULL,0,0,'1.0000','1.0000','10.0000','10.0000',NULL,'0.0000','0.3400','0.3400','0.0000','0.0000','0.0000','10.0000','10.0000','0.0000','1.0000','configurable',NULL,NULL,NULL,NULL,NULL,'10.0000','10.0000','10.0000','10.0000','0.0000','0.0000',NULL,0,NULL,NULL,NULL,NULL,NULL,NULL,NULL,NULL,NULL),('6','1','2020-02-01 02:17:19','2020-02-01 02:17:19','2',1,'5',0,'Test_Product-Red','Test Product-Red',NULL,NULL,NULL,0,0,'1.0000','1.0000','10.0000','0.0000',NULL,'0.0000','0.0000','0.0000','0.0000','0.0000','0.0000','0.0000','0.0000','0.0000','0.0000','simple',NULL,NULL,NULL,NULL,NULL,NULL,NULL,NULL,NULL,NULL,NULL,NULL,0,NULL,NULL,NULL,NULL,NULL,NULL,NULL,NULL,NULL);

Here, we can see discounts for the 2 items of $0.33 and $0.34 instead of $0.50 for each.

[m2-assistant]

Hi @shikhamis11. Thank you for working on this issue.
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