Django模型related_name格式化字符错误求助

问题场景 #

运行python manage.py makemigrations命令时触发如下错误：

ValueError: unsupported format character '_' (0x5f) at index 22

相关模型代码：

class ChangingDocument(PrivateCommentMixin,
                       models.Model):

    # Список изменяющих документов.
    treaty = models.ForeignKey("treaties.Treaty",
                               on_delete=models.CASCADE,
                               blank=False,
                               null=False,
                               related_name="%(app_label)s_%(class)s_related",
                               related_query_name="%(app_label)s_%(class)ss", )

    changing_document = models.ForeignKey("treaties.Treaty",
                                          on_delete=models.CASCADE,
                                          blank=False,
                                          null=False,
                                          related_name="%(app_label)s_%(class)_changing_documents_related",
                                          related_query_name="%(app_label)s_%(class)ss_changing_documents", )

    def __str__(self):
        return str(self.changing_document)

错误堆栈信息：

(venv) michael@michael:/media/michael/750/Programming/treaty_outer/treaty_project$ python manage.py makemigrations
Traceback (most recent call last):
  File "manage.py", line 22, in <module>
    main()
  File "manage.py", line 18, in main
    execute_from_command_line(sys.argv)
  File "/media/michael/750/Programming/treaty_outer/venv/lib/python3.8/site-packages/django/core/management/__init__.py", line 442, in execute_from_command_line
    utility.execute()
  File "/media/michael/750/Programming/treaty_outer/venv/lib/python3.8/site-packages/django/core/management/__init__.py", line 416, in execute
    django.setup()
  File "/media/michael/750/Programming/treaty_outer/venv/lib/python3.8/site-packages/django/__init__.py", line 24, in setup
    apps.populate(settings.INSTALLED_APPS)
  File "/media/michael/750/Programming/treaty_outer/venv/lib/python3.8/site-packages/django/apps/registry.py", line 116, in populate
    app_config.import_models()
  File "/media/michael/750/Programming/treaty_outer/venv/lib/python3.8/site-packages/django/apps/config.py", line 269, in import_models
    self.models_module = import_module(models_module_name)
  File "/usr/lib/python3.8/importlib/__init__.py", line 127, in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
  File "<frozen importlib._bootstrap>", line 1014, in _gcd_import
  File "<frozen importlib._bootstrap>", line 991, in _find_and_load
  File "<frozen importlib._bootstrap>", line 975, in _find_and_load_unlocked
  File "<frozen importlib._bootstrap>", line 671, in _load_unlocked
  File "<frozen importlib._bootstrap_external>", line 848, in exec_module
  File "<frozen importlib._bootstrap>", line 219, in _call_with_frames_removed
  File "/media/michael/750/Programming/treaty_outer/treaty_project/changing_documents/models.py", line 6, in <module>
    class ChangingDocument(PrivateCommentMixin,
  File "/media/michael/750/Programming/treaty_outer/venv/lib/python3.8/site-packages/django/db/models/base.py", line 194, in __new__
    new_class.add_to_class(obj_name, obj)
  File "/media/michael/750/Programming/treaty_outer/venv/lib/python3.8/site-packages/django/db/models/base.py", line 371, in add_to_class
    value.contribute_to_class(cls, name)
  File "/media/michael/750/Programming/treaty_outer/venv/lib/python3.8/site-packages/django/db/models/fields/related.py", line 866, in contribute_to_class
    super().contribute_to_class(cls, name, private_only=private_only, **kwargs)
  File "/media/michael/750/Programming/treaty_outer/venv/lib/python3.8/site-packages/django/db/models/fields/related.py", line 357, in contribute_to_class
    related_name %= {
ValueError: unsupported format character '_' (0x5f) at index 22

错误原因 #

问题出在changing_document字段的related_name参数中：

related_name="%(app_label)s_%(class)_changing_documents_related"

这里的%(class)缺少了格式化后缀s，导致Python字符串格式化引擎将%(class)_解析为：占位符%(class)后跟格式字符_，但_不是合法的Python字符串格式符，因此抛出错误。

修复方案 #

将related_name中的%(class)改为%(class)s，同时可优化related_query_name的复数形式（可选）：

修复后的模型代码：

class ChangingDocument(PrivateCommentMixin,
                       models.Model):

    # Список изменяющих документов.
    treaty = models.ForeignKey("treaties.Treaty",
                               on_delete=models.CASCADE,
                               blank=False,
                               null=False,
                               related_name="%(app_label)s_%(class)s_related",
                               related_query_name="%(app_label)s_%(class)s", )

    changing_document = models.ForeignKey("treaties.Treaty",
                                          on_delete=models.CASCADE,
                                          blank=False,
                                          null=False,
                                          related_name="%(app_label)s_%(class)s_changing_documents_related",
                                          related_query_name="%(app_label)s_%(class)s_changing_documents", )

    def __str__(self):
        return str(self.changing_document)

验证步骤 #

1. 保存修改后的models.py文件
2. 重新运行迁移命令：

   python manage.py makemigrations
   python manage.py migrate
   

3. 确认错误不再出现，迁移正常完成

内容的提问来源于stack exchange，提问作者Kifsif