我在确定如何在提交表单时自动为ForeignKey字段创建模型实例时遇到了很多麻烦。这是一个简单的玩具网站，说明了这个问题：
我有两个型号，Model1和Model2。 Model2包含一个到Model1的ForeignKey。我希望用户能够通过专门选择要存储在ForeignKey中的Model1实例来创建Model2的实例，或者将该值留空并让自动生成Model1的实例。
这就是我觉得代码应该是这样的。我的models.py代码非常简单：
# models.py
from django.db import models
from django.core.validators import MinValueValidator
class Model1(models.Model):
# Note this field cannot be negative
my_field1 = models.IntegerField(validators=[MinValueValidator(0)])
class Model2(models.Model):
# blank = True will make key_to_model1 not required on the form,
# but since null = False, I will still require the ForeignKey
# to be set in the database.
related_model1 = models.ForeignKey(Model1, blank=True)
# Note this field cannot be negative
my_field2 = models.IntegerField(validators=[MinValueValidator(0)])
forms.py有点涉及，但是发生的事情非常简单。如果Model2Form没有收到Model1的实例，它会尝试在clean方法中自动创建一个，验证它，如果它有效，它会保存它。如果它无效，则会引发异常。
#forms.py
from django import forms
from django.forms.models import model_to_dict
from .models import Model1, Model2
# A ModelForm used for validation purposes only.
class Model1Form(forms.ModelForm):
class Meta:
model = Model1
class Model2Form(forms.ModelForm):
class Meta:
model = Model2
def clean(self):
cleaned_data = super(Model2Form, self).clean()
if not cleaned_data.get('related_model1', None):
# Don't instantiate field2 if it doesn't exist.
val = cleaned_data.get('my_field2', None)
if not val:
raise forms.ValidationError("My field must exist")
# Generate a new instance of Model1 based on Model2's data
new_model1 = Model1(my_field1=val)
# validate the Model1 instance with a form form
validation_form_data = model_to_dict(new_model1)
validation_form = Model1Form(validation_form_data)
if not validation_form.is_valid():
raise forms.ValidationError("Could not create a proper instance of Model1.")
# set the model1 instance to the related model and save it to the database.
new_model1.save()
cleaned_data['related_model1'] = new_model1
return cleaned_data
但是，这种方法不起作用。如果我在表单中输入有效数据，它可以正常工作。但是，如果我没有为ForeignKey输入任何内容并为整数设置负值，我会得到一个ValueError。
回溯：文件 “/Library/Python/2.7/site-packages/django/core/handlers/base.py”in get_response 111. response = callback（request，* callback_args，** callback_kwargs）文件“/Library/Python/2.7/site-packages/django/views/generic/base.py”in 视图 48.返回self.dispatch（request，* args，** kwargs）文件“/Library/Python/2.7/site-packages/django/views/generic/base.py”in 调度 69.返回处理程序（request，* args，** kwargs）文件“/Library/Python/2.7/site-packages/django/views/generic/edit.py”in 岗位 172. return super（BaseCreateView，self）.post（request，* args，** kwargs）file“/Library/Python/2.7/site-packages/django/views/generic/edit.py”in 岗位 137.如果form.is_valid（）：文件“/Library/Python/2.7/site-packages/django/forms/forms.py”in is_valid 124.返回self.is_bound而不是bool（self.errors）文件“/Library/Python/2.7/site-packages/django/forms/forms.py”in _get_errors 115. self.full_clean（）文件“/Library/Python/2.7/site-packages/django/forms/forms.py”in full_clean 272. self._post_clean（）文件“/Library/Python/2.7/site-packages/django/forms/models.py”in _post_clean 309. self.instance = construct_instance（self，self.instance，opts.fields，opts.exclude）文件 “/Library/Python/2.7/site-packages/django/forms/models.py”in construct_instance 51. f.save_form_data（instance，cleaning_data [f.name]）文件 “/Library/Python/2.7/site-packages/django/db/models/fields/init.py” 在save_form_data中 454. setattr（instance，self.name，data）File“/Library/Python/2.7/site-packages/django/db/models/fields/related.py” 在设置 362.（instance._meta.object_name，self.field.name））
异常类型：ValueError at / add / Exception值：无法分配 无：“Model2.related_model1”不允许空值。
所以，正在发生的事情是Django正在捕获我的ValidationError，并且即使验证失败，仍然会创建Model2的实例。
我可以通过覆盖_post_clean方法来修复此问题，以便在出现错误时不创建Model2的实例。但是，这个解决方案很难看。特别是，_post_clean的行为通常非常有用 - 在更复杂的项目中，我需要_post_clean来运行其他原因。
我还可以允许ForeignKey为null但在实践中从不将其设置为null。但是，再次，这似乎是一个坏主意。
我甚至可以设置一个虚拟的Model1，只要对尝试的新Model1进行验证失败，我就会使用它，但这似乎也是hackish。
总的来说，我可以想到很多黑客来解决这个问题，但我不知道如何以一种相当干净，pythonic的方式解决这个问题。
答案 0 :(得分：1)
我找到了一个我认为可以接受的解决方案，基于karthikr在评论中的讨论。我当然仍然愿意接受替代方案。
我们的想法是在视图中使用逻辑来选择两种形式进行验证：一种形式是标准模型形式，一种是没有ForeignKey字段的模型形式。
所以，我的models.py是完全相同的。
我的forms.py有两个Model2表单...一个非常简单，一个没有ForeignKey字段，并且有新的逻辑为ForeignKey动态生成Model1的新实例。新表单的干净逻辑只是我过去在Model2Form中使用的干净逻辑：
#forms.py
from django import forms
from django.forms.models import model_to_dict
from .models import Model1, Model2
# A ModelForm used for validation purposes only.
class Model1Form(forms.ModelForm):
class Meta:
model = Model1
class Model2Form(forms.ModelForm):
class Meta:
model = Model2
# This inherits from Model2Form so that any additional logic that I put in Model2Form
# will apply to it.
class Model2FormPrime(Model2Form):
class Meta:
model = Model2
exclude = ('related_model1',)
def clean(self):
cleaned_data = super(Model2Form, self).clean()
if cleaned_data.get('related_model1', None):
raise Exception('Huh? This should not happen...')
# Don't instantiate field2 if it doesn't exist.
val = cleaned_data.get('my_field2', None)
if not val:
raise forms.ValidationError("My field must exist")
# Generate a new instance of Model1 based on Model2's data
new_model1 = Model1(my_field1=val)
# validate the Model1 instance with a form form
validation_form_data = model_to_dict(new_model1)
validation_form = Model1Form(validation_form_data)
if not validation_form.is_valid():
raise forms.ValidationError("Could not create a proper instance of Model1.")
# set the Model1 instance to the related model and save it to the database.
cleaned_data['related_model1'] = new_model1
return cleaned_data
def save(self, commit=True):
# Best to wait til save is called to save the instance of Model1
# so that instances aren't created when the Model2Form is invalid
self.cleaned_data['related_model1'].save()
# Need to handle saving this way because otherwise related_model1 is excluded
# from the save due to Meta.excludes
instance = super(Model2FormPrime, self).save(False)
instance.related_model1 = self.cleaned_data['related_model1']
instance.save()
return instance
然后我的视图逻辑使用两种形式之一进行验证，具体取决于发布数据。如果它使用Model2FormPrime并且验证失败，它会将数据和错误移动到常规Model2Form以显示用户：
# Create your views here.
from django.views.generic.edit import CreateView
from django.http import HttpResponseRedirect
from .forms import Model2Form, Model2FormPrime
class Model2CreateView(CreateView):
form_class = Model2Form
template_name = 'form_template.html'
success_url = '/add/'
def post(self, request, *args, **kwargs):
if request.POST.get('related_model', None):
# Complete data can just be sent to the standard CreateView form
return super(Model2CreateView, self).post(request, *args, **kwargs)
else:
# super does this, and I won't be calling super.
self.object = None
# use Model2FormPrime to validate the post data without the related model.
validation_form = Model2FormPrime(request.POST)
if validation_form.is_valid():
return self.form_valid(validation_form)
else:
# Create a normal instance of Model2Form to be displayed to the user
# Insantiate it with post data and validation_form's errors
form = Model2Form(request.POST)
form._errors = validation_form._errors
return self.form_invalid(form)
此解决方案有效，而且非常灵活。我可以为我的模型和基础Model2Form添加逻辑，而不必担心破坏它或违反DRY。
但是，它有点难看，因为它要求我使用两种形式来完成一项工作，在表格之间传递错误。所以，如果有人可以提出任何建议，我肯定会接受其他解决方案。