Django中使用cleaned_data更新/保存表单字段失效问题

我希望将表单中的TOTAL字段设置为计算得出的PRE-TOTAL值，但出现异常：已通过cleaned_data更新了form_data中的total值，却无法将该值保存至数据库，该字段仍显示默认值。

相关代码 #

MODELS.PY #

from django.db import models
from django.utils import timezone

class av_pomades(models.Model):
    cod_prod = models.IntegerField(primary_key=True)
    name = models.CharField(max_length=100)
    price = models.FloatField(default=0)
    pic = models.ImageField(default='default.jpg', blank=True)

    def __str__(self):
        return self.name


pomades_to_chose = (
    ('Suavecito - water based','Suavecito - water based'),
    ('Reuzel Blue - water based','Reuzel Blue - water based'),
    ('Layrite - water based','Layrite - water based'),
    ('Suavecito - oil based','Suavecito - oil based'),
    ('Reuzel Pink - Oil Based','Reuzel Pink - Oil Based'),
    ('Layrite - oil based','Layrite - oil based')
)

class order_form(models.Model):
    pomade = models.CharField(max_length=100, choices=pomades_to_chose)
    amount = models.PositiveIntegerField()
    ord_date = models.DateField(default=timezone.now)
    total = models.FloatField(default=0)

    def __str__(self):
        return self.pomade

FORMS.PY #

from dataclasses import fields
from tabnanny import verbose
from django import forms
from . import models

class order_forms(forms.ModelForm):
    class Meta:
        # model that the fields will come from
        model = models.order_form
        # chosing the fields to be displayed
        fields = ['pomade', 'amount', 'total']  

VIEWS.PY #

from django.shortcuts import render, redirect
from .models import av_pomades, order_form
from django.http import HttpResponse
from django.contrib.auth.decorators import login_required
from . import forms

@login_required(login_url='accounts:login')
def purchase(request):
    if request.method == 'POST':
        form = forms.order_forms(request.POST)
        if form.is_valid():
            # creating a cleanned data instace
            form_data = form.cleaned_data
            ch_pomade = str(form_data['pomade'])

            # retrieving the right price from the selected product 
            pomades_prices = av_pomades.objects.all().values('name','price')        
            for pomades_data in pomades_prices:
                if ch_pomade in pomades_data['name']:
                    ch_pomade_price = pomades_data['price']

            # setting the total price
            famount = int(form_data['amount'])
            pre_total = ch_pomade_price * famount
            
            # updating the price in the for to be recorded in DB
            
            # HERE IS THE PROBLEMATIC PART I GUESS
            form_data['total'] = pre_total
            print(form_data)           
            

            form.save()
            return redirect('products:order_success')
    else:
        form = forms.order_forms()
    return render(request,'basquet.html', {'order_form':form})

问题原因与解决方法 #

原因 #

form.cleaned_data是表单验证后生成的数据副本，修改它并不会影响表单绑定的模型实例。form.save()最终保存的是表单内部的instance对象，不是你修改后的form_data。

解决方法1：修改表单实例后保存（快速修复） #

在视图中，不要修改cleaned_data，而是通过commit=False获取未保存的实例，修改字段后再保存：

# 替换原视图中修改form_data和save的代码
instance = form.save(commit=False)
# 计算total
famount = int(form_data['amount'])
pre_total = ch_pomade_price * famount
instance.total = pre_total
instance.save()

解决方法2：将计算逻辑移到表单的clean方法（更规范） #

把total的计算逻辑放到表单的clean方法中，符合Django表单验证的流程，同时自动同步到实例：
修改FORMS.PY：

from django import forms
from . import models

class order_forms(forms.ModelForm):
    class Meta:
        model = models.order_form
        fields = ['pomade', 'amount', 'total']  

    def clean(self):
        cleaned_data = super().clean()
        pomade_name = cleaned_data.get('pomade')
        amount = cleaned_data.get('amount')
        
        if pomade_name and amount:
            # 优化查询：直接通过name获取产品，避免遍历所有数据
            try:
                pomade = models.av_pomades.objects.get(name=pomade_name)
                total = pomade.price * amount
                cleaned_data['total'] = total
                # 将计算值同步到表单实例，确保保存时生效
                self.instance.total = total
            except models.av_pomades.DoesNotExist:
                self.add_error('pomade', '所选产品不存在于数据库中')
        
        return cleaned_data

此时VIEWS.PY可以简化，不用再计算total，直接保存即可：

@login_required(login_url='accounts:login')
def purchase(request):
    if request.method == 'POST':
        form = forms.order_forms(request.POST)
        if form.is_valid():
            form.save()
            return redirect('products:order_success')
    else:
        form = forms.order_forms()
    return render(request,'basquet.html', {'order_form':form})

额外优化：查询效率提升 #

原视图中遍历所有av_pomades数据来匹配价格，效率极低。上面的解决方法2中用get直接查询，能大幅提升性能，同时还能处理产品不存在的异常情况。

内容的提问来源于stack exchange，提问作者IARC