我已经创建了一个模型,我正在为它渲染默认/未修改的模型表单。这就产生了64个SQL查询,因为它有很多外键,而这些外键又有更多的外键。
是否可以强制它始终(默认情况下)每次返回其中一个模型时执行select_related
?
答案 0 :(得分:45)
您可以创建自定义管理器,只需覆盖get_queryset
即可在任何地方应用。例如:
class MyManager(models.Manager):
def get_queryset(self):
return super(MyManager, self).get_queryset().select_related('foo', 'bar')
(在Django 1.6之前,它是get_query_set
)。
答案 1 :(得分:36)
这也是一个有趣的伎俩:
class DefaultSelectOrPrefetchManager(models.Manager):
def __init__(self, *args, **kwargs):
self._select_related = kwargs.pop('select_related', None)
self._prefetch_related = kwargs.pop('prefetch_related', None)
super(DefaultSelectOrPrefetchManager, self).__init__(*args, **kwargs)
def get_queryset(self, *args, **kwargs):
qs = super(DefaultSelectOrPrefetchManager, self).get_queryset(*args, **kwargs)
if self._select_related:
qs = qs.select_related(*self._select_related)
if self._prefetch_related:
qs = qs.prefetch_related(*self._prefetch_related)
return qs
class Sandwich(models.Model):
bread = models.ForeignKey(Bread)
extras = models.ManyToManyField(Extra)
# ...
objects = DefaultSelectOrPrefetchManager(select_related=('bread',), prefetch_related=('extras',))
然后,您可以在模型类之间轻松地重新使用管理器。作为一个示例用例,如果您在模型上有一个__unicode__
方法,它会生成一个字符串,其中包含来自相关模型的某些信息(或其他任何意味着相关模型几乎总是必填)。
...如果你真的想要变得古怪,这里有一个更通用的版本。它允许您使用args
或kwargs
的任意组合调用默认查询集上的任何方法序列。代码中可能存在一些错误,但您明白了。
from django.db import models
class MethodCalls(object):
"""
A mock object which logs chained method calls.
"""
def __init__(self):
self._calls = []
def __getattr__(self, name):
c = Call(self, name)
self._calls.append(c)
return c
def __iter__(self):
for c in self._calls:
yield tuple(c)
class Call(object):
"""
Used by `MethodCalls` objects internally to represent chained method calls.
"""
def __init__(self, calls_obj, method_name):
self._calls = calls_obj
self.method_name = method_name
def __call__(self, *method_args, **method_kwargs):
self.method_args = method_args
self.method_kwargs = method_kwargs
return self._calls
def __iter__(self):
yield self.method_name
yield self.method_args
yield self.method_kwargs
class DefaultQuerysetMethodCallsManager(models.Manager):
"""
A model manager class which allows specification of a sequence of
method calls to be applied by default to base querysets.
`DefaultQuerysetMethodCallsManager` instances expose a property
`default_queryset_method_calls` to which chained method calls can be
applied to indicate which methods should be called on base querysets.
"""
def __init__(self, *args, **kwargs):
self.default_queryset_method_calls = MethodCalls()
super(DefaultQuerysetMethodCallsManager, self).__init__(*args, **kwargs)
def get_queryset(self, *args, **kwargs):
qs = super(DefaultQuerysetMethodCallsManager, self).get_queryset(*args, **kwargs)
for method_name, method_args, method_kwargs in self.default_queryset_method_calls:
qs = getattr(qs, method_name)(*method_args, **method_kwargs)
return qs
class Sandwich(models.Model):
bread = models.ForeignKey(Bread)
extras = models.ManyToManyField(Extra)
# Other field definitions...
objects = DefaultQuerysetMethodCallsManager()
objects.default_queryset_method_calls.filter(
bread__type='wheat',
).select_related(
'bread',
).prefetch_related(
'extras',
)
python-mock-inspired MethodCalls
对象试图使API更自然。有些人可能会觉得有点混乱。如果是这样,你可以为__init__
arg或kwarg提供那些只接受方法调用信息元组的代码。
答案 2 :(得分:2)
创建自定义models.Manager
并覆盖所有方法(filter
,get
等)并将select_related附加到每个查询中。然后将此管理器设置为模型上的objects
属性。
我建议您只需查看代码并在需要的地方添加select_related
,因为在所有内容上执行select_related会导致一些严重的性能问题(并且不会完全清楚它会在哪里发生)从)。