Django 在同一模型中引用 ManyToManyField 错误，没有引号

Question

https://docs.djangoproject.com/en/1.11/topics/db/examples/many_to_many/

所以我的代码基于这个文档，我遇到了一个错误。

from django.db import models
class Gift(models.Model):
name = models.CharField(
max_length=120, unique=True, help_text='Enter the gift item name'
)
# skipping to the ManyToMany ....
genre = models.ManyToManyField(Category)
class Category(models.Model):
# skipping stuff like CATEGORIES ...
type = models.IntegerField(
primary_key   = True, choices = CATEGORIES, default = EVENT,
)
description = models.CharField(max_length=500, null=True, blank=True)

我收到此错误

Traceback (most recent call last):
File "manage.py", line 22, in <module>
execute_from_command_line(sys.argv)
File "/.../gifting_db/lib/python3.6/site-packages/django/core/management/__init__.py", line 363, in execute_from_command_line
utility.execute()
File "/.../gifting_db/lib/python3.6/site-packages/django/core/management/__init__.py", line 337, in execute
django.setup()
File "/.../gifting_db/lib/python3.6/site-packages/django/__init__.py", line 27, in setup
apps.populate(settings.INSTALLED_APPS)
File "/.../gifting_db/lib/python3.6/site-packages/django/apps/registry.py", line 108, in populate
app_config.import_models()
File "/.../gifting_db/lib/python3.6/site-packages/django/apps/config.py", line 202, in import_models
self.models_module = import_module(models_module_name)
File "/.../gifting_db/lib/python3.6/importlib/__init__.py", line 126, in import_module
return _bootstrap._gcd_import(name[level:], package, level)
File "<frozen importlib._bootstrap>", line 978, in _gcd_import
File "<frozen importlib._bootstrap>", line 961, in _find_and_load
File "<frozen importlib._bootstrap>", line 950, in _find_and_load_unlocked
File "<frozen importlib._bootstrap>", line 655, in _load_unlocked
File "<frozen importlib._bootstrap_external>", line 678, in exec_module
File "<frozen importlib._bootstrap>", line 205, in _call_with_frames_removed
File "/.../gifting_db/gifting/gifts/models.py", line 5, in <module>
class Gift(models.Model):
File "/.../gifting_db/gifting/gifts/models.py", line 20, in Gift
category    = models.ManyToManyField(Category)
NameError: name 'Category' is not defined

但是，如果我将引号添加到"类别"中，它就可以了。知道为什么吗？django 文档示例不使用在"发布"和"文章"之间 https://docs.djangoproject.com/en/1.11/topics/db/examples/many_to_many/引号。如果有的话，我想我将"类别"作为字符串放入，而不是作为对类别对象的引用？

genre = models.ManyToManyField('Category')

Answer 1

您需要在Gift之前定义Category或使用引号。

from django.db import models

class Category(models.Model):
# skipping stuff like CATEGORIES ...
type = models.IntegerField(
primary_key = True, choices=CATEGORIES, default=EVENT)
description = models.CharField(max_length=500, null=True, blank=True)

class Gift(models.Model):
name = models.CharField(max_length=120, unique=True, help_text='Enter the gift item name'
)
# skipping to the ManyToMany ....
genre = models.ManyToManyField(Category)

或use quotes

from django.db import models

class Gift(models.Model):
name = models.CharField(max_length=120, unique=True, help_text='Enter the gift item name'
)
# skipping to the ManyToMany ....
genre = models.ManyToManyField('Category')

class Category(models.Model):
# skipping stuff like CATEGORIES ...
type = models.IntegerField(primary_key=True, choices=CATEGORIES, default=EVENT,)
description = models.CharField(max_length=500, null=True, blank=True

Answer 2

您的问题是直到Gift之后才定义Category。如果您想在没有引号的情况下使用它，则需要将Category放在Gift上方。否则，报价单工作正常并提供相同的功能。

https://docs.djangoproject.com/en/dev/ref/models/fields/#foreignkey

如果需要在尚未创建的模型上创建关系 定义时，您可以使用模型的名称，而不是模型 对象本身：