使用元类动态扩展 Django 模型
Dynamically extending django models using a metaclass
问题
我正在尝试构建一个自动扩展模型 classes 具有额外字段的框架。以下是我正在尝试做的事情的简短摘要：
给定一个模型class
class Pizza(models.Model):
name = models.CharField(max_length=10)
price = models.DecimalField(max_digits=10, decimal_places=2)
我想自动生成一个 class，每个 class 字段都有一个附加字段，生成一个 class 类似于以下内容：
class PizzaGenerated(models.Model):
name = models.CharField(max_length=10)
name_new = models.CharField(max_length=10)
price = models.DecimalField(max_digits=10, decimal_places=2)
price_new = models.DecimalField(max_digits=10, decimal_places=2)
如您所见，对于 Pizza 的每个属性，都添加了一个带有 _new 后缀的附加字段。
无论模型的结构如何，我都需要我的解决方案。特别是，我正在寻找一种允许复制 ForeignKey-Fields
的方法
我的方法
上面扩展 Pizza class 的例子可以用下面的代码解决：
class ResMetaclass(models.base.ModelBase):
def __new__(cls, name, bases, attrs):
fields = {
k: v for k, v in attrs.items() if not k.startswith('_') and isinstance(v, models.Field)
}
attrs_extended = {
**attrs,
**{fieldname + '_new': fieldtype.clone() for fieldname, fieldtype in fields.items()}
}
bases = (models.Model,)
clsobj = super().__new__(cls, name, bases, attrs_extended)
return clsobj
class EntityBase(models.Model, metaclass=ResMetaclass):
class Meta:
abstract = True
class Pizza(EntityBase):
name = models.CharField(max_length=10)
price = models.DecimalField(max_digits=10, decimal_places=2)
Pizza class 已由 EntityMetaclass 元 class 成功扩展。
问题所在
不幸的是，当模型包含 ForeignKey-Field 时，上述代码失败，产生以下回溯：
File "manage.py", line 17, in main
execute_from_command_line(sys.argv)
File "/home/niklas/dev/pyflx/venv/lib64/python3.7/site-packages/django/core/management/__init__.py", line 381, in execute_from_command_line
utility.execute()
File "/home/niklas/dev/pyflx/venv/lib64/python3.7/site-packages/django/core/management/__init__.py", line 357, in execute
django.setup()
File "/home/niklas/dev/pyflx/venv/lib64/python3.7/site-packages/django/__init__.py", line 24, in setup
apps.populate(settings.INSTALLED_APPS)
File "/home/niklas/dev/pyflx/venv/lib64/python3.7/site-packages/django/apps/registry.py", line 114, in populate
app_config.import_models()
File "/home/niklas/dev/pyflx/venv/lib64/python3.7/site-packages/django/apps/config.py", line 211, in import_models
self.models_module = import_module(models_module_name)
File "/usr/lib64/python3.7/importlib/__init__.py", line 127, in import_module
return _bootstrap._gcd_import(name[level:], package, level)
File "<frozen importlib._bootstrap>", line 1006, in _gcd_import
File "<frozen importlib._bootstrap>", line 983, in _find_and_load
File "<frozen importlib._bootstrap>", line 967, in _find_and_load_unlocked
File "<frozen importlib._bootstrap>", line 677, in _load_unlocked
File "<frozen importlib._bootstrap_external>", line 728, in exec_module
File "<frozen importlib._bootstrap>", line 219, in _call_with_frames_removed
File "/home/niklas/dev/pyflx/mig/models.py", line 20, in <module>
class Contact(EntityBase):
File "/home/niklas/dev/pyflx/pyflx/models.py", line 17, in __new__
**{fieldname + '_patch_value': fieldtype.clone() for fieldname, fieldtype in fields.items() if fieldname != 'id'},
File "/home/niklas/dev/pyflx/pyflx/models.py", line 17, in <dictcomp>
**{fieldname + '_patch_value': fieldtype.clone() for fieldname, fieldtype in fields.items() if fieldname != 'id'},
File "/home/niklas/dev/pyflx/venv/lib64/python3.7/site-packages/django/db/models/fields/__init__.py", line 492, in clone
name, path, args, kwargs = self.deconstruct()
File "/home/niklas/dev/pyflx/venv/lib64/python3.7/site-packages/django/db/models/fields/related.py", line 856, in deconstruct
name, path, args, kwargs = super().deconstruct()
File "/home/niklas/dev/pyflx/venv/lib64/python3.7/site-packages/django/db/models/fields/related.py", line 583, in deconstruct
swappable_setting = self.swappable_setting
File "/home/niklas/dev/pyflx/venv/lib64/python3.7/site-packages/django/db/models/fields/related.py", line 374, in swappable_setting
return apps.get_swappable_settings_name(to_string)
File "/home/niklas/dev/pyflx/venv/lib64/python3.7/site-packages/django/apps/registry.py", line 288, in get_swappable_settings_name
for model in self.get_models(include_swapped=True):
File "/home/niklas/dev/pyflx/venv/lib64/python3.7/site-packages/django/apps/registry.py", line 178, in get_models
self.check_models_ready()
File "/home/niklas/dev/pyflx/venv/lib64/python3.7/site-packages/django/apps/registry.py", line 140, in check_models_ready
raise AppRegistryNotReady("Models aren't loaded yet.")
django.core.exceptions.AppRegistryNotReady: Models aren't loaded yet.
有办法解决这个问题吗？
棘手！
问题在于，当您尝试 .clone 模型中的字段时，它们尚未完全初始化，因此，Django 机制通过其字符串 qualifeid 名称查找 foreign-model无法使用。问题是代码不检查外国模型是否作为 class 引用而不是字符串传递。
解决此问题的唯一方法似乎是 monkey-patching 在创建 classes 时进行这些检查。
同时，在克隆 foreign-object 字段时，related_field - 由 django ORM 自动创建的反向引用，因此可以从 "pointed-at" 对象获取 "holder" 对象必须显式传递给新的、克隆的字段。否则它会指向原始字段。
这需要更多的 monkeypatching，在 .clone 调用的内部工作中插入一个显式的 "related_name" 参数。
这 2 件事正在完成，似乎有效。这是我使用的代码，基于您的代码：
from django.db import models
from django.db.models.fields import related
from unittest.mock import patch
class ResMetaclass(models.base.ModelBase):
def __new__(cls, name, bases, attrs):
fields = {
k: v for k, v in attrs.items() if not k.startswith('_') and isinstance(v, models.Field)
}
new_fields = {}
for field_name, field in fields.items():
new_field_name = field_name + "_new"
if not isinstance(field, related.RelatedField):
new_fields[new_field_name] = field.clone()
else:
real_deconstruct = field.deconstruct
def _deconstruct():
name, path, args, kwargs = real_deconstruct()
kwargs["related_name"] = new_field_name
return name, path, args, kwargs
with patch("django.apps.registry.apps.check_models_ready", lambda: True):
field.deconstruct = _deconstruct
# Assume foregnKeys are always within the same file, and
# disable model-ready checking:
new_fields[new_field_name] = field.clone()
del field.deconstruct
attrs_extended = {
**attrs,
**new_fields
}
bases = (models.Model,)
clsobj = super().__new__(cls, name, bases, attrs_extended)
return clsobj
class EntityBase(models.Model, metaclass=ResMetaclass):
class Meta:
abstract = True
class Pizza(EntityBase):
name = models.CharField(max_length=10)
price = models.DecimalField(max_digits=10, decimal_places=2)
class MenuEntry(EntityBase):
entry_number = models.IntegerField()
pizza = models.ForeignKey("Pizza", on_delete="cascade")
以及 MenuEntry class 上的结果字段：
In [1]: from test1.models import Pizza, MenuEntry
In [2]: MenuEntry._meta.fields
Out[2]:
(<django.db.models.fields.AutoField: id>,
<django.db.models.fields.IntegerField: entry_number>,
<django.db.models.fields.related.ForeignKey: pizza>,
<django.db.models.fields.IntegerField: entry_number_new>,
<django.db.models.fields.related.ForeignKey: pizza_new>)
问题
我正在尝试构建一个自动扩展模型 classes 具有额外字段的框架。以下是我正在尝试做的事情的简短摘要：
给定一个模型class
class Pizza(models.Model):
name = models.CharField(max_length=10)
price = models.DecimalField(max_digits=10, decimal_places=2)
我想自动生成一个 class，每个 class 字段都有一个附加字段，生成一个 class 类似于以下内容：
class PizzaGenerated(models.Model):
name = models.CharField(max_length=10)
name_new = models.CharField(max_length=10)
price = models.DecimalField(max_digits=10, decimal_places=2)
price_new = models.DecimalField(max_digits=10, decimal_places=2)
如您所见，对于 Pizza 的每个属性，都添加了一个带有 _new 后缀的附加字段。
无论模型的结构如何，我都需要我的解决方案。特别是，我正在寻找一种允许复制 ForeignKey-Fields
我的方法
上面扩展 Pizza class 的例子可以用下面的代码解决：
class ResMetaclass(models.base.ModelBase):
def __new__(cls, name, bases, attrs):
fields = {
k: v for k, v in attrs.items() if not k.startswith('_') and isinstance(v, models.Field)
}
attrs_extended = {
**attrs,
**{fieldname + '_new': fieldtype.clone() for fieldname, fieldtype in fields.items()}
}
bases = (models.Model,)
clsobj = super().__new__(cls, name, bases, attrs_extended)
return clsobj
class EntityBase(models.Model, metaclass=ResMetaclass):
class Meta:
abstract = True
class Pizza(EntityBase):
name = models.CharField(max_length=10)
price = models.DecimalField(max_digits=10, decimal_places=2)
Pizza class 已由 EntityMetaclass 元 class 成功扩展。
问题所在
不幸的是，当模型包含 ForeignKey-Field 时，上述代码失败，产生以下回溯：
File "manage.py", line 17, in main
execute_from_command_line(sys.argv)
File "/home/niklas/dev/pyflx/venv/lib64/python3.7/site-packages/django/core/management/__init__.py", line 381, in execute_from_command_line
utility.execute()
File "/home/niklas/dev/pyflx/venv/lib64/python3.7/site-packages/django/core/management/__init__.py", line 357, in execute
django.setup()
File "/home/niklas/dev/pyflx/venv/lib64/python3.7/site-packages/django/__init__.py", line 24, in setup
apps.populate(settings.INSTALLED_APPS)
File "/home/niklas/dev/pyflx/venv/lib64/python3.7/site-packages/django/apps/registry.py", line 114, in populate
app_config.import_models()
File "/home/niklas/dev/pyflx/venv/lib64/python3.7/site-packages/django/apps/config.py", line 211, in import_models
self.models_module = import_module(models_module_name)
File "/usr/lib64/python3.7/importlib/__init__.py", line 127, in import_module
return _bootstrap._gcd_import(name[level:], package, level)
File "<frozen importlib._bootstrap>", line 1006, in _gcd_import
File "<frozen importlib._bootstrap>", line 983, in _find_and_load
File "<frozen importlib._bootstrap>", line 967, in _find_and_load_unlocked
File "<frozen importlib._bootstrap>", line 677, in _load_unlocked
File "<frozen importlib._bootstrap_external>", line 728, in exec_module
File "<frozen importlib._bootstrap>", line 219, in _call_with_frames_removed
File "/home/niklas/dev/pyflx/mig/models.py", line 20, in <module>
class Contact(EntityBase):
File "/home/niklas/dev/pyflx/pyflx/models.py", line 17, in __new__
**{fieldname + '_patch_value': fieldtype.clone() for fieldname, fieldtype in fields.items() if fieldname != 'id'},
File "/home/niklas/dev/pyflx/pyflx/models.py", line 17, in <dictcomp>
**{fieldname + '_patch_value': fieldtype.clone() for fieldname, fieldtype in fields.items() if fieldname != 'id'},
File "/home/niklas/dev/pyflx/venv/lib64/python3.7/site-packages/django/db/models/fields/__init__.py", line 492, in clone
name, path, args, kwargs = self.deconstruct()
File "/home/niklas/dev/pyflx/venv/lib64/python3.7/site-packages/django/db/models/fields/related.py", line 856, in deconstruct
name, path, args, kwargs = super().deconstruct()
File "/home/niklas/dev/pyflx/venv/lib64/python3.7/site-packages/django/db/models/fields/related.py", line 583, in deconstruct
swappable_setting = self.swappable_setting
File "/home/niklas/dev/pyflx/venv/lib64/python3.7/site-packages/django/db/models/fields/related.py", line 374, in swappable_setting
return apps.get_swappable_settings_name(to_string)
File "/home/niklas/dev/pyflx/venv/lib64/python3.7/site-packages/django/apps/registry.py", line 288, in get_swappable_settings_name
for model in self.get_models(include_swapped=True):
File "/home/niklas/dev/pyflx/venv/lib64/python3.7/site-packages/django/apps/registry.py", line 178, in get_models
self.check_models_ready()
File "/home/niklas/dev/pyflx/venv/lib64/python3.7/site-packages/django/apps/registry.py", line 140, in check_models_ready
raise AppRegistryNotReady("Models aren't loaded yet.")
django.core.exceptions.AppRegistryNotReady: Models aren't loaded yet.
有办法解决这个问题吗？
棘手！
问题在于，当您尝试 .clone 模型中的字段时，它们尚未完全初始化，因此，Django 机制通过其字符串 qualifeid 名称查找 foreign-model无法使用。问题是代码不检查外国模型是否作为 class 引用而不是字符串传递。
解决此问题的唯一方法似乎是 monkey-patching 在创建 classes 时进行这些检查。
同时，在克隆 foreign-object 字段时，related_field - 由 django ORM 自动创建的反向引用，因此可以从 "pointed-at" 对象获取 "holder" 对象必须显式传递给新的、克隆的字段。否则它会指向原始字段。
这需要更多的 monkeypatching，在 .clone 调用的内部工作中插入一个显式的 "related_name" 参数。
这 2 件事正在完成，似乎有效。这是我使用的代码，基于您的代码：
from django.db import models
from django.db.models.fields import related
from unittest.mock import patch
class ResMetaclass(models.base.ModelBase):
def __new__(cls, name, bases, attrs):
fields = {
k: v for k, v in attrs.items() if not k.startswith('_') and isinstance(v, models.Field)
}
new_fields = {}
for field_name, field in fields.items():
new_field_name = field_name + "_new"
if not isinstance(field, related.RelatedField):
new_fields[new_field_name] = field.clone()
else:
real_deconstruct = field.deconstruct
def _deconstruct():
name, path, args, kwargs = real_deconstruct()
kwargs["related_name"] = new_field_name
return name, path, args, kwargs
with patch("django.apps.registry.apps.check_models_ready", lambda: True):
field.deconstruct = _deconstruct
# Assume foregnKeys are always within the same file, and
# disable model-ready checking:
new_fields[new_field_name] = field.clone()
del field.deconstruct
attrs_extended = {
**attrs,
**new_fields
}
bases = (models.Model,)
clsobj = super().__new__(cls, name, bases, attrs_extended)
return clsobj
class EntityBase(models.Model, metaclass=ResMetaclass):
class Meta:
abstract = True
class Pizza(EntityBase):
name = models.CharField(max_length=10)
price = models.DecimalField(max_digits=10, decimal_places=2)
class MenuEntry(EntityBase):
entry_number = models.IntegerField()
pizza = models.ForeignKey("Pizza", on_delete="cascade")
以及 MenuEntry class 上的结果字段：
In [1]: from test1.models import Pizza, MenuEntry
In [2]: MenuEntry._meta.fields
Out[2]:
(<django.db.models.fields.AutoField: id>,
<django.db.models.fields.IntegerField: entry_number>,
<django.db.models.fields.related.ForeignKey: pizza>,
<django.db.models.fields.IntegerField: entry_number_new>,
<django.db.models.fields.related.ForeignKey: pizza_new>)