如何使用相同模型的其他字段的值在Django模型中创建一个字段？

Question

I wants to create a field name: total which have the total price of all the products*quqantity. What I want is how can I do total = price*quantity in django model. As you can see there can be more than one products.
I have join the OderItem model with Order through tabularinline.

 class OrderItemInline(admin.TabularInline): model = OrderItem raw_id_fields = ['product'] class OrderAdmin(admin.ModelAdmin): list_display = ['id','name', 'mobile_no','address','status', 'created'] list_editable = ['status'] list_per_page = 15 list_filter = ['status'] search_fields = ('id',) inlines = [OrderItemInline] admin.site.register(Order, OrderAdmin) 

 class Order(models.Model): name = models.CharField(max_length=60) email = models.EmailField(max_length=60,default=None, blank=True) mobile_no = models.CharField(max_length=13, default=None) address = models.CharField(max_length=150) created = models.DateTimeField(auto_now_add=True) updated = models.DateTimeField(auto_now=True) status_choices = ( ('In Queue', 'In Queue'), ('Processing', 'Processing'), ('Ready', 'Ready'), ('Delivered', 'Delivered'), ('Paid', 'Paid'), ('Cancelled', 'Cancelled'), ) status = models.CharField(max_length=15, choices=status_choices, default=status_choices[0][0]) class Meta: ordering = ('created', ) def __str__(self): return 'Order {}'.format(self.id) def get_total_cost(self): return sum(item.get_cost() for item in self.items.all()) class OrderItem(models.Model): order = models.ForeignKey(Order, related_name='items', on_delete=models.CASCADE) product = models.ForeignKey(MenuVariant, related_name='order_items', on_delete=models.CASCADE, default=None) size = models.CharField(max_length=20, default=None) price = models.DecimalField(max_digits=10, decimal_places=2) quantity = models.PositiveIntegerField(default=1) # total = models.DecimalField(max_digits=10, decimal_places=2,default=0) def __str__(self): return '{}'.format(self.id) def get_cost(self): return self.price * self.quantity 

Answer 1

You can get the total price for an Order directly with a database query, so there's no need to save it in the database:

from django.db.models import Sum, F, FloatField
orders = Order.objects.annotate(total=Sum(F('items__quantity')*F('items__price'), output_field=FloatField()))

This adds a total field to each result of the query. So then:

price_of_first_order = orders.first().total

And if you want to display the total for an order in the admin, subclass the ModelAdmin and add get_total_cost to the readonly_fields , as explained here :

A read-only field can not only display data from a model's field, it can also display the output of a model's method or a method of the ModelAdmin class itself. This is very similar to the way ModelAdmin.list_display behaves.

Answer 2

While I, like dirkgroten don't quite see the point of saving the value in a model field, you could add a field total=models.IntegerField(null=True, blank=True) to your Order model and use a post_save to update the value.

So, since you already have a get_total_cost() we can do something like:

def order_pre_save(sender, instance, **kwargs):
    instance.total_cost = instance.get_total_cost()

pre_save.connect(order_pre_save, sender=Order)

EDIT: As dirkgroten points out you'll need the post_save on the OrderItem :

def order_item_pre_save(sender, instance, **kwargs):
    instance.order.total_cost = instance.order.get_total_cost()
    instance.order.save()

post_save.connect(order_item_pre_save, sender=OrderItem)

And you'd also need to update the value on delete:

def order_item_pre_delete(sender, instance, **kwargs):
    instance.order.total_cost = instance.order.get_total_cost() - instance.get_cost()
    instance.order.save()

post_delete.connect(order_item_pre_delete, sender=OrderItem)