检查Django中的OneToOneField是否为None

时间：2010-08-11 22:08:29

标签： python django-models one-to-one

我有两个这样的模型：

class Type1Profile(models.Model):
    user = models.OneToOneField(User, unique=True)
    ...


class Type2Profile(models.Model):
    user = models.OneToOneField(User, unique=True)
    ...

如果用户具有Type1或Type2配置文件，我需要执行某些操作：

if request.user.type1profile != None:
    # do something
elif request.user.type2profile != None:
    # do something else
else:
    # do something else

但是，对于没有type1或type2配置文件的用户，执行这样的代码会产生以下错误：

Type1Profile matching query does not exist.

如何查看用户的个人资料类型？

由于

8 个答案:

答案 0 :(得分：78)

要检查（OneToOne）关系是否存在，您可以使用hasattr函数：

if hasattr(request.user, 'type1profile'):
    # do something
elif hasattr(request.user, 'type2profile'):
    # do something else
else:
    # do something else

答案 1 :(得分：42)

通过测试None ness的模型上的相应字段，可以看出特定模型的可以为空的一对一关系是否为空，但是如果你只测试 测试一对一关系起源的模型。例如，给定这两个类...... 

class Place(models.Model):
    name = models.CharField(max_length=50)
    address = models.CharField(max_length=80)

class Restaurant(models.Model):  # The class where the one-to-one originates
    place = models.OneToOneField(Place, blank=True, null=True)
    serves_hot_dogs = models.BooleanField()
    serves_pizza = models.BooleanField()

...要查看Restaurant是否有Place，我们可以使用以下代码：

>>> r = Restaurant(serves_hot_dogs=True, serves_pizza=False)
>>> r.save()
>>> if r.place is None:
>>>    print "Restaurant has no place!"
Restaurant has no place!

要查看Place是否有Restaurant，了解引用restaurant实例上的Place属性会引发Restaurant.DoesNotExist异常非常重要如果没有相应的餐厅。这是因为Django使用QuerySet.get()在内部执行查找。例如：

>>> p2 = Place(name='Ace Hardware', address='1013 N. Ashland')
>>> p2.save()
>>> p2.restaurant
Traceback (most recent call last):
    ...
DoesNotExist: Restaurant matching query does not exist.

在这种情况下，Occam的剃须刀占优势，确定Place是否有Restautrant的最佳方法是标准try / {{1}构造如here所述。

except

虽然joctee建议使用>>> try: >>> restaurant = p2.restaurant >>> except Restaurant.DoesNotExist: >>> print "Place has no restaurant!" >>> else: >>> # Do something with p2's restaurant here. 在实践中起作用，但它实际上只是偶然起作用，因为hasattr抑制所有例外（包括hasattr）而不是只是DoesNotExist s，就像它应该的那样。正如Piet Delport指出的那样，这个行为实际上已经在Python 3.2中通过以下票证得到纠正：http://bugs.python.org/issue9666。此外 - 并且冒着听起来有意见的风险 - 我相信上面的AttributeError / try结构更能代表Django的工作方式，而使用except可以为新手提供可能的问题。制造FUD并传播坏习惯。

答案 2 :(得分：14)

我喜欢joctee's answer，因为它很简单。

if hasattr(request.user, 'type1profile'):
    # do something
elif hasattr(request.user, 'type2profile'):
    # do something else
else:
    # do something else

其他评论者提出担心它可能不适用于某些版本的Python或Django，但the Django documentation将此技术显示为其中一个选项：

  

您还可以使用hasattr来避免异常捕获的需要：

>>> hasattr(p2, 'restaurant')
False

当然，文档还显示了异常捕获技术：

  

p2没有相关的餐厅：

>>> from django.core.exceptions import ObjectDoesNotExist
>>> try:
>>>     p2.restaurant
>>> except ObjectDoesNotExist:
>>>     print("There is no restaurant here.")
There is no restaurant here.

我同意Joshua，抓住异常会使发生的事情更加清晰，但对我来说似乎更加混乱。也许这是一个合理的妥协？

>>> print(Restaurant.objects.filter(place=p2).first())
None

这只是按地点查询Restaurant个对象。如果该地方没有餐厅，则返回None。

这是一个可执行代码段供您玩这些选项。如果您安装了Python，Django和SQLite3，它应该只运行。我用Python 2.7，Python 3.4，Django 1.9.2和SQLite3 3.8.2测试了它。

# Tested with Django 1.9.2
import sys

import django
from django.apps import apps
from django.apps.config import AppConfig
from django.conf import settings
from django.core.exceptions import ObjectDoesNotExist
from django.db import connections, models, DEFAULT_DB_ALIAS
from django.db.models.base import ModelBase

NAME = 'udjango'


def main():
    setup()

    class Place(models.Model):
        name = models.CharField(max_length=50)
        address = models.CharField(max_length=80)

        def __str__(self):              # __unicode__ on Python 2
            return "%s the place" % self.name

    class Restaurant(models.Model):
        place = models.OneToOneField(Place, primary_key=True)
        serves_hot_dogs = models.BooleanField(default=False)
        serves_pizza = models.BooleanField(default=False)

        def __str__(self):              # __unicode__ on Python 2
            return "%s the restaurant" % self.place.name

    class Waiter(models.Model):
        restaurant = models.ForeignKey(Restaurant)
        name = models.CharField(max_length=50)

        def __str__(self):              # __unicode__ on Python 2
            return "%s the waiter at %s" % (self.name, self.restaurant)

    syncdb(Place)
    syncdb(Restaurant)
    syncdb(Waiter)

    p1 = Place(name='Demon Dogs', address='944 W. Fullerton')
    p1.save()
    p2 = Place(name='Ace Hardware', address='1013 N. Ashland')
    p2.save()
    r = Restaurant(place=p1, serves_hot_dogs=True, serves_pizza=False)
    r.save()

    print(r.place)
    print(p1.restaurant)

    # Option 1: try/except
    try:
        print(p2.restaurant)
    except ObjectDoesNotExist:
        print("There is no restaurant here.")

    # Option 2: getattr and hasattr
    print(getattr(p2, 'restaurant', 'There is no restaurant attribute.'))
    if hasattr(p2, 'restaurant'):
        print('Restaurant found by hasattr().')
    else:
        print('Restaurant not found by hasattr().')

    # Option 3: a query
    print(Restaurant.objects.filter(place=p2).first())


def setup():
    DB_FILE = NAME + '.db'
    with open(DB_FILE, 'w'):
        pass  # wipe the database
    settings.configure(
        DEBUG=True,
        DATABASES={
            DEFAULT_DB_ALIAS: {
                'ENGINE': 'django.db.backends.sqlite3',
                'NAME': DB_FILE}},
        LOGGING={'version': 1,
                 'disable_existing_loggers': False,
                 'formatters': {
                    'debug': {
                        'format': '%(asctime)s[%(levelname)s]'
                                  '%(name)s.%(funcName)s(): %(message)s',
                        'datefmt': '%Y-%m-%d %H:%M:%S'}},
                 'handlers': {
                    'console': {
                        'level': 'DEBUG',
                        'class': 'logging.StreamHandler',
                        'formatter': 'debug'}},
                 'root': {
                    'handlers': ['console'],
                    'level': 'WARN'},
                 'loggers': {
                    "django.db": {"level": "WARN"}}})
    app_config = AppConfig(NAME, sys.modules['__main__'])
    apps.populate([app_config])
    django.setup()
    original_new_func = ModelBase.__new__

    @staticmethod
    def patched_new(cls, name, bases, attrs):
        if 'Meta' not in attrs:
            class Meta:
                app_label = NAME
            attrs['Meta'] = Meta
        return original_new_func(cls, name, bases, attrs)
    ModelBase.__new__ = patched_new


def syncdb(model):
    """ Standard syncdb expects models to be in reliable locations.

    Based on https://github.com/django/django/blob/1.9.3
    /django/core/management/commands/migrate.py#L285
    """
    connection = connections[DEFAULT_DB_ALIAS]
    with connection.schema_editor() as editor:
        editor.create_model(model)

main()

答案 3 :(得分：9)

如何使用try / except块？

def get_profile_or_none(user, profile_cls):

    try:
        profile = getattr(user, profile_cls.__name__.lower())
    except profile_cls.DoesNotExist:
        profile = None

    return profile

然后，像这样使用！

u = request.user
if get_profile_or_none(u, Type1Profile) is not None:
    # do something
elif get_profile_or_none(u, Type2Profile) is not None:
    # do something else
else:
    # d'oh!

我想你可以使用它作为泛型函数来获取任何反向的OneToOne实例，给定一个原始类（这里：你的配置文件类）和一个相关实例（这里：request.user）。

答案 4 :(得分：3)

使用select_related！

>>> user = User.objects.select_related('type1profile').get(pk=111)
>>> user.type1profile
None

答案 5 :(得分：0)

我正在使用has_attr的组合，但是没有：

class DriverLocation(models.Model):
    driver = models.OneToOneField(Driver, related_name='location', on_delete=models.CASCADE)

class Driver(models.Model):
    pass

    @property
    def has_location(self):
        return not hasattr(self, "location") or self.location is None

答案 6 :(得分：0)

如果您有模型

class UserProfile(models.Model):
    user = models.OneToOneField(User, unique=True)

您只需要让任何用户知道UserProfile存在/不存在-从数据库的角度来看，最有效的方法可以使用 exists查询。

现有查询将仅返回布尔值，而不是像hasattr(request.user, 'type1profile')这样的反向属性访问-它将生成 get查询并返回完整的对象表示形式

要这样做-您需要向用户模型添加属性

class User(AbstractBaseUser)

@property
def has_profile():
    return UserProfile.objects.filter(user=self.pk).exists()

答案 7 :(得分：0)

智能方法之一是添加 custom 字段 OneToOneOrNoneField 并使用 [适用于Django > = 1.9] 

from django.db.models.fields.related_descriptors import ReverseOneToOneDescriptor
from django.core.exceptions import ObjectDoesNotExist
from django.db import models


class SingleRelatedObjectDescriptorReturnsNone(ReverseOneToOneDescriptor):
    def __get__(self, *args, **kwargs):
        try:
            return super().__get__(*args, **kwargs)
        except ObjectDoesNotExist:
            return None


class OneToOneOrNoneField(models.OneToOneField):
    """A OneToOneField that returns None if the related object doesn't exist"""
    related_accessor_class = SingleRelatedObjectDescriptorReturnsNone

    def __init__(self, *args, **kwargs):
        kwargs.setdefault('null', True)
        kwargs.setdefault('blank', True)
        super().__init__(*args, **kwargs)

实施 

class Restaurant(models.Model):  # The class where the one-to-one originates
    place = OneToOneOrNoneField(Place)
    serves_hot_dogs = models.BooleanField()
    serves_pizza = models.BooleanField()

用法 

r = Restaurant(serves_hot_dogs=True, serves_pizza=False)
r.place  # will return None
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