django rest framework 是否从数据库中查询单个实例的所有对象?
Does django rest framework query all objects from database for a single instance?
使用 RetrieveAPIView 时必须定义 queryset,通常定义为 queryset = <Model>
.objects.all()
为什么检索单个实例需要加载所有对象?
您没有加载所有对象。查询集 未 评估。 RetrieveAPIView
[classy-drf] 将使用查询集作为基础进行查询。事实上,在.retrieve(…)
方法中,它会获取对象:
def retrieve(self, request, *args, **kwargs):
instance = self.get_object()
serializer = self.get_serializer(instance)
return Response(serializer.data)
和 .get_object(…)
方法,它将获取单个对象:
def get_object(self):
"""
Returns the object the view is displaying.
You may want to override this if you need to provide non-standard
queryset lookups. Eg if objects are referenced using multiple
keyword arguments in the url conf.
"""
queryset = self.filter_queryset(self.get_queryset())
# Perform the lookup filtering.
lookup_url_kwarg = self.lookup_url_kwarg or self.lookup_field
assert lookup_url_kwarg in self.kwargs, (
'Expected view %s to be called with a URL keyword argument '
'named "%s". Fix your URL conf, or set the `.lookup_field` '
'attribute on the view correctly.' %
(self.__class__.__name__, lookup_url_kwarg)
)
filter_kwargs = {self.lookup_field: self.kwargs[lookup_url_kwarg]}
obj = get_object_or_404(queryset, **filter_kwargs)
# May raise a permission denied
self.check_object_permissions(self.request, obj)
return obj
因此,例如,它会归结为 get_object_or_404(MyModel.objects.all(), pk=42)
,这将进行查询以获取具有主键 42
的单个记录。 QuerySet
是惰性的:MyModel.objects.all()
是 不 评估,除非查询集被消耗,但这里它因此用于构造一个新的查询集,最终将获取一个单条记录。
使用 RetrieveAPIView 时必须定义 queryset,通常定义为 queryset = <Model>
.objects.all()
为什么检索单个实例需要加载所有对象?
您没有加载所有对象。查询集 未 评估。 RetrieveAPIView
[classy-drf] 将使用查询集作为基础进行查询。事实上,在.retrieve(…)
方法中,它会获取对象:
def retrieve(self, request, *args, **kwargs): instance = self.get_object() serializer = self.get_serializer(instance) return Response(serializer.data)
和 .get_object(…)
方法,它将获取单个对象:
def get_object(self): """ Returns the object the view is displaying. You may want to override this if you need to provide non-standard queryset lookups. Eg if objects are referenced using multiple keyword arguments in the url conf. """ queryset = self.filter_queryset(self.get_queryset()) # Perform the lookup filtering. lookup_url_kwarg = self.lookup_url_kwarg or self.lookup_field assert lookup_url_kwarg in self.kwargs, ( 'Expected view %s to be called with a URL keyword argument ' 'named "%s". Fix your URL conf, or set the `.lookup_field` ' 'attribute on the view correctly.' % (self.__class__.__name__, lookup_url_kwarg) ) filter_kwargs = {self.lookup_field: self.kwargs[lookup_url_kwarg]} obj = get_object_or_404(queryset, **filter_kwargs) # May raise a permission denied self.check_object_permissions(self.request, obj) return obj
因此,例如,它会归结为 get_object_or_404(MyModel.objects.all(), pk=42)
,这将进行查询以获取具有主键 42
的单个记录。 QuerySet
是惰性的:MyModel.objects.all()
是 不 评估,除非查询集被消耗,但这里它因此用于构造一个新的查询集,最终将获取一个单条记录。