首先，我想纠正另一个家伙谁回答有关related_name，它应该这样做：
添加related_name以便更好地查询：

class PurchaseItem(models.Model):

    product = models.ForeignKey(
        Item, on_delete=models.CASCADE, related_name="purchases"
    ) # here
    quantity = models.PositiveSmallIntegerField()
    purchase_price = models.DecimalField(max_digits=6, decimal_places=2)
    paid_amount = models.DecimalField(max_digits=6, decimal_places=2)
    date_created = models.DateTimeField(auto_now_add=True)

现在，从Item示例中，您可以获取所有以“product”作为Item示例的PurchaseItem示例，例如：

foo = Item.objects.get(title="FOO") # getting FOO

foo_purchases = foo.purchases.all() # getting all PurchaseItem objects that has "foo" as being the "product" attribute

但您不需要它来创建signals ...您需要做的只是：

from django.db.models.signals import post_save, post_delete
from django.dispatch import receiver

@receiver(post_save, sender=PurchaseItem)
def update_item_inventory_on_create(sender, instance, created, **kwargs):

    """
    Everytime you create a PurchaseItem it will decrease the Item used as "Product" inventory
    based on the PurchaseItem quantity.

    Example: If my Product had 10 samples on inventory, and i create a PurchaseItem with 2 as quantity,
    after this signal is called, my product will now have only 8 samples on inventory.
    """

    if created:
        purchase_item_created = instance
        purchase_item_created.product.inventory -= purchase_item_created.quantity
        purchase_item_created.product.save()

@receiver(post_delete, sender=PurchaseItem)
def update_item_inventory_on_delete(sender, instance, **kwargs):
    """
    Everytime you delete a PurchaseItem it will increase the Item used as "Product" inventory
    based on the PurchaseItem quantity.

    Example: If my Product had 8 samples on inventory, and i have deleted a PurchaseItem with 2 as quantity,
    after this signal is called, my product will now have 10 samples on inventory.
    """

    purchase_item_created = instance
    purchase_item_created.product.inventory += purchase_item_created.quantity
    purchase_item_created.product.save()

请参考文档以获得更好的解释，我尽了最大努力使其简洁，并在信号文档字符串上进行了解释。
但是，使用该代码，您将能够在PurchaseItem创建时减少项目库存，并在PurchaseItem删除时增加项目库存

您可以使用signals或：
首先编辑PurchaseItem模型，并将related_name添加到product字段：

class PurchaseItem(models.Model):
    product = models.ForeignKey(Item,related_name="item", on_delete=models.CASCADE)
    quantity = models.PositiveSmallIntegerField()
    purchase_price = models.DecimalField(max_digits=6, decimal_places=2)
    paid_amount = models.DecimalField(max_digits=6, decimal_places=2)
    date_created = models.DateTimeField(auto_now_add=True)

现在，当您创建采购项目时：

purchase_item = PurchaseItem.objects.create(...)
purchase_item.item.inventory += purchase_item.quantity # or any number you want
purchase_item.item.save()

但尽量使用信号。

使用Django，有三种方法：
1.创建一个每当创建PurchaseItem时都会调用的信号接收器，该信号可以调用处理更新逻辑的方法。
1.重写PurchaseItem的save()方法，检查是否正在创建或更新对象，然后调用另一个处理更新逻辑的方法。
1.您也可以有一个视图调用更新逻辑，但是您可能希望尽可能地将业务隐藏在视图之外。
希望这对你有帮助