django - 在 Django 中注释外键字段
问题描述
假设我有三个模型:
class Product(models.Model):
name = models.CharField(max_length=255)
class Plan(models.Model):
products = models.ManyToManyField(
Product,
)
class User(models.Model):
plan = models.ForeignKey(Plan, on_delete=models.CASCADE)
我想进行查询以获取用户列表,预取他们的计划,并用产品计数注释他们的计划。我正在尝试这样的事情:
plan_queryset = Plan.objects.all().annotate(num_product=Count('products'));
queryset = Plan.user_set \
.prefetch_related(
Prefetch('plan',
queryset=plan_queryset
)
)
serializer = UserSerializer(queryset, many=True)
但不幸的是,计划查询集没有执行。将查询集传递到序列化程序中,我收到一条错误消息,指出“num_product”未定义。
在不对每个对象进行查询的情况下注释外键字段的最佳方法是什么?(n + 1 个问题)。
更新:
示例序列化程序:
class PlanSerializer(serializers.ModelSerializer):
num_product=serializers.IntegerField()
class Meta:
model = Plan
fields = ('num_product',)
class UserSerializer(serializers.ModelSerializer):
plan = PlanSerializer(read_only=True)
class Meta:
model = User
fields = ('plan',)
追溯:
Traceback (most recent call last):
File "/usr/local/lib/python3.6/site-packages/rest_framework/fields.py", line 441, in get_attribute
return get_attribute(instance, self.source_attrs)
File "/usr/local/lib/python3.6/site-packages/rest_framework/fields.py", line 100, in get_attribute
instance = getattr(instance, attr)
AttributeError: 'Plan' object has no attribute 'num_product'
During handling of the above exception, another exception occurred:
Traceback (most recent call last):
File "/usr/local/lib/python3.6/site-packages/django/core/handlers/exception.py", line 35, in inner
response = get_response(request)
File "/usr/local/lib/python3.6/site-packages/django/core/handlers/base.py", line 128, in _get_response
response = self.process_exception_by_middleware(e, request)
File "/usr/local/lib/python3.6/site-packages/django/core/handlers/base.py", line 126, in _get_response
response = wrapped_callback(request, *callback_args, **callback_kwargs)
File "/usr/local/lib/python3.6/site-packages/django/views/decorators/csrf.py", line 54, in wrapped_view
return view_func(*args, **kwargs)
File "/usr/local/lib/python3.6/site-packages/django/views/generic/base.py", line 69, in view
return self.dispatch(request, *args, **kwargs)
File "/usr/local/lib/python3.6/site-packages/rest_framework/views.py", line 489, in dispatch
response = self.handle_exception(exc)
File "/usr/local/lib/python3.6/site-packages/rest_framework/views.py", line 449, in handle_exception
self.raise_uncaught_exception(exc)
File "/usr/local/lib/python3.6/site-packages/rest_framework/views.py", line 486, in dispatch
response = handler(request, *args, **kwargs)
File "/usr/local/lib/python3.6/site-packages/rest_framework/generics.py", line 201, in get
return self.list(request, *args, **kwargs)
File "/usr/local/lib/python3.6/site-packages/rest_framework/mixins.py", line 45, in list
return self.get_paginated_response(serializer.data)
File "/usr/local/lib/python3.6/site-packages/rest_framework/serializers.py", line 738, in data
ret = super(ListSerializer, self).data
File "/usr/local/lib/python3.6/site-packages/rest_framework/serializers.py", line 262, in data
self._data = self.to_representation(self.instance)
File "/usr/local/lib/python3.6/site-packages/rest_framework/serializers.py", line 656, in to_representation
self.child.to_representation(item) for item in iterable
File "/usr/local/lib/python3.6/site-packages/rest_framework/serializers.py", line 656, in <listcomp>
self.child.to_representation(item) for item in iterable
File "/usr/local/lib/python3.6/site-packages/rest_framework/serializers.py", line 500, in to_representation
ret[field.field_name] = field.to_representation(attribute)
File "/usr/local/lib/python3.6/site-packages/rest_framework/serializers.py", line 487, in to_representation
attribute = field.get_attribute(instance)
File "/usr/local/lib/python3.6/site-packages/rest_framework/fields.py", line 460, in get_attribute
raise type(exc)(msg)
AttributeError: Got AttributeError when attempting to get a value for field `num_product` on serializer `PlanSerializer`.
The serializer field might be named incorrectly and not match any attribute or key on the `Plan` instance.
Original exception text was: 'Plan' object has no attribute 'num_product'.
解决方案
弄清楚了。该错误实际上在于查询的设置方式:
plan = Plan.objects.get(pk=plan_pk)
queryset = plan.user_set \
.prefetch_related(
Prefetch('plan',
queryset=plan_queryset
)
)
使用plan.user_set
而不是直接在用户对象管理器上查询,意味着 Django 使用的是来自 plan.user_set的缓存计划对象,而不是带有注释的预取查询。解决此问题的两种方法:
1)设置查询以使用用户对象管理器:
queryset = User.objects.filter(plan__pk=plan_pk) \
.prefetch_related(
Prefetch('plan',
queryset=plan_queryset
)
)
2) 对原始的 Plan.objects.get() 调用进行注释,使其具有必要的字段。
长话短说:检查您的查询并确保模型没有从以前的调用中缓存!