Django - 在查询中加入 3 个模型

Question

class Item(models.Model):
    CATEGORY = (
        ('Gudang Kering', 'Gudang Kering'),
        ('Gudang Basah','Gudang Basah'),
        )
    name = models.CharField(max_length=200,null= True)
    stock = models.IntegerField(default='0', blank=False, null=True)
    category = models.CharField(max_length=200,null= True,choices=CATEGORY)
    description = models.CharField(max_length=200,null= True, blank= True)
    date_created = models.DateTimeField(auto_now_add= True)
    tags = models.ManyToManyField(Tag)

class Issue(models.Model):
    STATUS = (
        ('Pending', 'Pending'),
        ('Granted','Granted'),
        ('Denied','Denied'),
        )
    customer = models.ForeignKey(Customer, null=True, on_delete= models.SET_NULL)
    item = models.ForeignKey(Item, null=True, on_delete= models.SET_NULL)
    quantity = models.IntegerField(default='0', blank=False, null=True)
    date_created = models.DateTimeField(auto_now_add=True, auto_now=False)
    status = models.CharField(max_length=200,null= True, choices=STATUS)


class Receive(models.Model):
    STATUS = (
        ('Pending', 'Pending'),
        ('Granted','Granted'),
        ('Denied','Denied'),
        )
    customer = models.ForeignKey(Customer, null=True, on_delete= models.SET_NULL)
    item = models.ForeignKey(Item, null=True, on_delete= models.SET_NULL)
    quantity = models.IntegerField(default='0', blank=False, null=True)
    date_created = models.DateTimeField(auto_now_add=True, auto_now=False)
    status = models.CharField(max_length=200,null= True, choices=STATUS)

“你好，对不起，我正在尝试学习 django 并想问，如何编写查询来计算对象，如果 2 具有相同的项目和状态授予，它将计算库存 - 数量（问题）+ 数量（接收)，我想将该值保存为数据库中库存的新值"

Answer 1

• 首先获取Item()和Customer()的对象，因为它们在Issue()中设置为外键
• 创建Issue()的对象并为字段赋值
• 要更新Item.stock，请使用我们已有的Issue.quantity 和Item.stock值
• 保存实例

# c_id and i_id should be fetched from post or any other way
customer_obj = Customer.objects.get(id=c_id)
item_obj = Item.objects.get(id=i_id)

# create an object of Issue and assign values
issue_obj = Issue()
issue_obj.customer = customer_obj 
issue_obj.item = item_obj
# like so assign values to other fields also and save Issue object
issue_obj.save()

# calculate stock and save the Item object
item_obj.stock = item_obj.stock - issue_obj.quantity
item_obj.save()

当使用外键时，应将模型类的对象分配给该字段。例如：customer = models.ForeignKey(Customer, null=True, on_delete= models.SET_NULL) 是Issue() 中的一个字段。为了保存Issue() 的实例，我们需要将Customer() 的对象分配给Issue 对象的customer 字段。

Answer 2

无需将相同的信息存储两次。它会使这个解决方案更加混乱。您可以在运行时添加这些值。例如：

解决方案一：你可以直接annotate查询集中数量的值。无需将附加信息存储在 Item 模型的 stock 属性中。例如：

from django.db.models import Q, F, Sum

items = Item.objects.annotate(issue_q=Sum('issue__quantity', filter=Q(issue__status="GRANTED")).annotate(total=F('stock')-F('issue_q'))

for item in items:
   print(item.total)

方案二：增加显示总值的属性方法：

class Item(models.Model):
   ...

   @property
   def total(self):
      return self.stock - self.item_set.filter(status="GRANTED").aggregate(quantity_s=Sum('quantity'))['quantity_s']


for item in Item.objects.all():
   print(item.total)

顺便说一句，这个解决方案的效率不如第一个，因为它会对 DB 进行多次查询。