如何使属性函数像数据库中的真实字段一样工作

Question

i want to make a (group by) which depend on some other fields ,

models.py

class Product(models.Model):
    pass


class Order(models.Model):
    id = models.AutoField(primary_key = True)
    products = models.ManyToManyField(Product ,through='ProductOrder') 
    date_time = models.DateTimeField(default=datetime.now()) 

    @property
    def total(self):
        return self.productorder_set.aggregate(
        price_sum=Sum(F('quantity') * F('product__price'), 
     output_field=IntegerField()) )['price_sum'] 



class ProductOrder(models.Model):
    product = models.ForeignKey(Product, 
    on_delete=models.CASCADE,default='' )
    ordering = models.ForeignKey(Order, 
    on_delete=models.CASCADE,blank=True,null=True)
    quantity = models.IntegerField(default=1)

views.py

class View(ListView):
pass

    def get_context_data(self , *args,**kwargs):
        context = super().get_context_data(*args , **kwargs)
        context['daily'] = 
    Order.objects.values('date_time').aggregate(days=Sum('total'))
    return context

(Cannot resolve keyword 'total' into field. Choices are:...)

is it possible to make the property function act as a field in the db ? or if there another way to achieve the same thing i trying to implement ? i also tried to views.py

from .models.Order import total 

context['daily'] = 
Order.objects.values('date_time').aggregate(days=Sum(total()))

but doesnt work !

thanks for any advice ..

Answer 1

You can annotate your queryset. For example:

Order.objects.annotate(
    
)

You can then for example annotate per date_time value, like:

Order.objects.values('date_time').annotate(
    _total=Sum(F('productorder__quantity') * F('productorder__product__price'))
).order_by('date_time')

If you want to do this per day, you can first make an annotation that extracts the day:

from django.db.models.functions import 

Order.objects.annotate(
    
).values('day').annotate(
    _total=Sum(F('productorder__quantity') * F('productorder__product__price'))
).order_by('day')

This is a QuerySet of dictionaries, that looks like:

<QuerySet [
    {'day': datetime(2019, 1, 1), '_total': 123},
    {'day': datetime(2019, 1, 2), '_total': 456},
    {'day': datetime(2019, 1, 3), '_total': 789},
]>