Django 1.7：懒惰的评估，以避免“应用程序注册表尚未准备好”在models.py [英] Django 1.7: Lazy evaluation to avoid "App registry isn't ready yet" in models.py

问题描述

我在模型的方法定义中有一个查询：

I have a query in a model's method definition:

'content_type_id': ContentType.objects.get_for_model(model).pk,

https://github.com/seperman/django-tagging/blob/develop/tagging/models.py#L107

它引发了应用程序注册表尚未准备好。

And it raises "App registry is not ready yet".

将行放在另一个独立的函数中没有帮助因为它仍然被调用。

Putting the line inside another independent function doesn't help either since it still gets called.

Django文档：

在导入时使用ORM执行数据库查询模型
模块也将触发此异常。 ORM无法正常运行
，直到所有型号都可用。

Executing database queries with the ORM at import time in models modules will also trigger this exception. The ORM cannot function properly until all models are available.

如何让它懒惰评估这一行？
我想到通过管理命令将其结果手动缓存到文件中。然后阅读缓存的结果，但应该有一个更好/更清洁的解决方案（？）

How can I make it lazy evaluate this line? I thought about caching its results manually in a file by making a management command. And then reading the cached result but there should be a much better/cleaner solution(?)

./manage.py shell
Traceback (most recent call last):
File "./manage.py", line 10, in <module>
  execute_from_command_line(sys.argv)
File "/var/www/something.com/something/shared/env/local/lib/python2.7/site-packages/django/core/management/__init__.py", line 385, in execute_from_command_line
  utility.execute()
File "/var/www/something.com/something/shared/env/local/lib/python2.7/site-packages/django/core/management/__init__.py", line 354, in execute
  django.setup()
File "/var/www/something.com/something/shared/env/local/lib/python2.7/site-packages/django/__init__.py", line 21, in setup
  apps.populate(settings.INSTALLED_APPS)
File "/var/www/something.com/something/shared/env/local/lib/python2.7/site-packages/django/apps/registry.py", line 108, in populate
  app_config.import_models(all_models)
File "/var/www/something.com/something/shared/env/local/lib/python2.7/site-packages/django/apps/config.py", line 202, in import_models
  self.models_module = import_module(models_module_name)
File "/usr/lib/python2.7/importlib/__init__.py", line 37, in import_module
  __import__(name)
File "/var/www/something.com/something/current/apps/riding/models.py", line 32, in <module>
  class RidingSection(CategoryBase):
File "/var/www/something.com/something/shared/env/local/lib/python2.7/site-packages/mptt/models.py", line 244, in __new__
  cls = meta.register(cls)
File "/var/www/something.com/something/shared/env/local/lib/python2.7/site-packages/mptt/models.py", line 325, in register
  obj = getattr(cls, attr)
File "/var/www/something.com/something/current/apps/tagging/fields.py", line 54, in __get__
  return edit_string_for_tags(Tag.objects.usage_for_model(owner))
File "/var/www/something.com/something/current/apps/tagging/models.py", line 148, in usage_for_model
  usage = self.usage_for_queryset(queryset, counts, min_count)
File "/var/www/something.com/something/current/apps/tagging/models.py", line 185, in usage_for_queryset
  return self._get_usage(queryset.model, counts, min_count, extra_joins, extra_criteria, params)
File "/var/www/something.com/something/current/apps/tagging/models.py", line 107, in _get_usage
  'content_type_id': ContentType.objects.get_for_model(model).pk,
File "/var/www/something.com/something/shared/env/local/lib/python2.7/site-packages/django/contrib/contenttypes/models.py", line 50, in get_for_model
  defaults={'name': smart_text(opts.verbose_name_raw)},
File "/var/www/something.com/something/shared/env/local/lib/python2.7/site-packages/django/db/models/manager.py", line 92, in manager_method
  return getattr(self.get_queryset(), name)(*args, **kwargs)
File "/var/www/something.com/something/shared/env/local/lib/python2.7/site-packages/django/db/models/query.py", line 422, in get_or_create
  return self.get(**lookup), False
File "/var/www/something.com/something/shared/env/local/lib/python2.7/site-packages/django/db/models/query.py", line 345, in get
  clone = self.filter(*args, **kwargs)
File "/var/www/something.com/something/shared/env/local/lib/python2.7/site-packages/django/db/models/query.py", line 691, in filter
  return self._filter_or_exclude(False, *args, **kwargs)
File "/var/www/something.com/something/shared/env/local/lib/python2.7/site-packages/django/db/models/query.py", line 709, in _filter_or_exclude
  clone.query.add_q(Q(*args, **kwargs))
File "/var/www/something.com/something/shared/env/local/lib/python2.7/site-packages/django/db/models/sql/query.py", line 1287, in add_q
  clause, require_inner = self._add_q(where_part, self.used_aliases)
File "/var/www/something.com/something/shared/env/local/lib/python2.7/site-packages/django/db/models/sql/query.py", line 1314, in _add_q
  current_negated=current_negated, connector=connector)
File "/var/www/something.com/something/shared/env/local/lib/python2.7/site-packages/django/db/models/sql/query.py", line 1138, in build_filter
  lookups, parts, reffed_aggregate = self.solve_lookup_type(arg)
File "/var/www/something.com/something/shared/env/local/lib/python2.7/site-packages/django/db/models/sql/query.py", line 1076, in solve_lookup_type
  _, field, _, lookup_parts = self.names_to_path(lookup_splitted, self.get_meta())
File "/var/www/something.com/something/shared/env/local/lib/python2.7/site-packages/django/db/models/sql/query.py", line 1339, in names_to_path
  field, model, direct, m2m = opts.get_field_by_name(name)
File "/var/www/something.com/something/shared/env/local/lib/python2.7/site-packages/django/db/models/options.py", line 416, in get_field_by_name
  cache = self.init_name_map()
File "/var/www/something.com/something/shared/env/local/lib/python2.7/site-packages/django/db/models/options.py", line 445, in init_name_map
  for f, model in self.get_all_related_m2m_objects_with_model():
File "/var/www/something.com/something/shared/env/local/lib/python2.7/site-packages/django/db/models/options.py", line 563, in get_all_related_m2m_objects_with_model
  cache = self._fill_related_many_to_many_cache()
File "/var/www/something.com/something/shared/env/local/lib/python2.7/site-packages/django/db/models/options.py", line 577, in _fill_related_many_to_many_cache
  for klass in self.apps.get_models():
File "/var/www/something.com/something/shared/env/local/lib/python2.7/site-packages/django/utils/lru_cache.py", line 101, in wrapper
  result = user_function(*args, **kwds)
File "/var/www/something.com/something/shared/env/local/lib/python2.7/site-packages/django/apps/registry.py", line 168, in get_models
  self.check_models_ready()
File "/var/www/something.com/something/shared/env/local/lib/python2.7/site-packages/django/apps/registry.py", line 131, in check_models_ready
  raise AppRegistryNotReady("Models aren't loaded yet.")

django.core.exceptions.AppRegistryNotReady：模型尚未加载。

django.core.exceptions.AppRegistryNotReady: Models aren't loaded yet.

推荐答案

django-tagging在这里有点不明智。

django-tagging is being a little inconsiderate here.

< a href =https://github.com/brosner/django-tagging/blob/master/tagging/fields.py#L36 =nofollow> TagField是一个描述符，这意味着如果您访问 YourModel.tags ，它运行执行数据库查询的代码。

TagField is a descriptor, meaning if you access YourModel.tags, it runs code which executes a database query.

只是这样，django-mptt迭代在启动期间（在应用程序缓存加载之前），您的MPTTModel类的所有属性，因此触发此异常。

It just so happens that django-mptt iterates over all the attributes of your MPTTModel classes during startup (before the app cache has loaded), so it happens to trigger this exception.

我不知道你会防止在django标记中维护API（ YourModel.tags 必须从 中获取标签）。这样的问题可能是django-tagging不再维护的原因。

I don't know how you'd prevent that in django-tagging while maintaining the API (YourModel.tags has to get tags from somewhere). Issues like this are probably the reason why django-tagging is no longer maintained.

但是，我是django-mptt的维护者，我有提交了解决方法。如果您升级到主版本的mptt，您应该会发现它现在可以工作。

However, I'm the maintainer of django-mptt and I've committed a workaround. If you upgrade to the master version of mptt you should find it works now.

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