HTML at /add_ TypeError /HTML

Question

在页面上获取此错误：

TypeError at /add_items/

str返回非字符串(NoneType类型)请求方法: GET请求URL：http://127.0.0.1:8000/add_items/ Django版本: 3.0.7异常类型: TypeError异常值：

str返回非字符串(NoneType类型)

意见：

def add_items(request):
    form = StockCreateForm(request.POST or None)
    if form.is_valid():
        form.save()
        messages.success(request, 'Successfully Saved')
        z=messages.success(request, 'Successfull')
        print(z)
        return redirect("admin.home")
    context = {
    "form": form,
    "title": "Add Item",
    }
    return render(request, "hdm_template/add_items.html", context)

Html页面：

{% extends 'hdm_template/base_template.html' %}

{% block page_title %}
    Add Item
{% endblock page_title %}

{% block main_content %}

{% load static %}
{% load crispy_forms_tags %}


<section class="content">
  <div class="wrapper">

      <div class="row">
          <div class="col-md-12">
              <!-- general form elements -->
              <div class="card card-primary">
              <div class="card-header">
                  <h3 class="card-title">Add Item</h3>
              </div>
              <!-- /.card-header -->
              <!-- form start -->
              <form method='POST' action=''>
                <div class="form-row">
                  <div class="form-group col-md-6 mb-0">
                    {{ form.category|as_crispy_field }}
                  </div>
                </div>
                <div class="form-row">
                  <div class="form-group col-md-6 mb-0">
                    {{ form.item_name|as_crispy_field }}
                  </div>
                  <div class="form-group col-md-2 mb-0">
                    {{ form.quantity|as_crispy_field }}
                  </div>
                </div>
                  <input class="btn btn-primary" type="submit" value='Save'>
                </form>
              </div>
              <!-- /.card -->

          </div>
      </div>

  </div><!-- /.container-fluid -->
</section>

URL：

path('add_items/',HDMViews.add_items,name='add_items'),

模型：

class Stock(models.Model):
    category = models.ForeignKey(Category, on_delete=models.CASCADE,blank=True )
    item_name = models.CharField(max_length=50, blank=True, null=True)
    quantity = models.IntegerField(default='0', blank=True, null=True)
    
    def __str__(self):
        return self.item_name

forms.py：

from django import forms
from django.forms import Form
from .models import *

class StockCreateForm(forms.ModelForm):
    class Meta:
        model = Stock
        fields = ['category', 'item_name', 'quantity']

    def clean_item_name(self):
        item_name = self.cleaned_data.get('item_name')
        if not category:
            raise forms.ValidationError('This field is required')

        for instance in Stock.objects.all():
            if instance.category == item_name:
                raise forms.ValidationError(str(item_name) + ' is already created')

        return category


    def clean_item_name(self):
        item_name = self.cleaned_data.get('item_name')
        if not item_name:
            raise forms.ValidationError('This field is required')
        return item_name

我不明白哪里出了问题

Answer 1

__str__必须返回一个字符串。在您的例子中，self.item_name是可空的。看起来这就是产生错误的原因。尝试将代码更改为如下内容：

def __str__(self):
    return self.item_name if self.item_name or str(self.pk)