Django -模型表单与ManyToManyField -如何计算字段?

Question

我有这个代码在我的views.py:

from django.shortcuts import render
from .helpers import *
from .forms import OrderForm
def create_order(request):
if request.method == 'POST':
form = OrderForm(request.POST)
if not request.POST._mutable:
request.POST._mutable = True

if form.is_valid():
obj = form.save(commit=False)
obj.total_price = calculate_total_price_obj(obj)
obj.save()
else:
form = OrderForm()
return render(request, 'create_order.html', {'form': form})

我有这个在我的helpers.py:

def calculate_total_price_obj(obj) -> float:
"""
Calculate the total_price of the order.
This function has to be called after the Order has been created and saved.
"""
prices_of_ordered_foods = obj.ordered_foods
print(prices_of_ordered_foods)
return sum(prices_of_ordered_foods)

我想要实现的是:

1. 获取用户发送的表单
2. 根据用户填写表格时选择的膳食/食品的价格计算字段total_price(这是我的models.py中的ManyToManyField)。计算完成后，保存表单。(计算为简单的sum()方法)

然而，这不起作用。我得到这个错误:<MyOrderObject> needs to have a value for field "id" before this many-to-many relationship can be used.

有什么好办法吗?

谢谢

Answer 1

不能用Order.save发出的post_save信号来计算total_price，因为只有在调用save方法之后才会设置多对多关系。因此，您需要保存Order实例，分配膳食/食物，然后才调用calculate_total_price_obj。

def create_order(request):
if request.method == 'POST':
form = OrderForm(request.POST)
if not request.POST._mutable:
request.POST._mutable = True

if form.is_valid():
obj = form.save(commit=True)
# Note that you must set many-to-many relationships before calling calculate_total_price_obj
obj.total_price = calculate_total_price_obj(obj)
obj.save()
else:
form = OrderForm()
return render(request, 'create_order.html', {'form': form})