在 Django 中注释外键字段

Question

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，并使用产品计数来注释他们的计划。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：

序列化程序示例：

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'.

Answer 1

想通了。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之前的调用中缓存！

Answer 2

如果没有完整的堆栈跟踪等，很难判断，但我认为问题出在：

class User(models.Model):
        plan = models.ForeignKey(Plan, on_delete=models.CASCADE)

你能帮我试试以下吗？

class User(models.Model):
        plan = models.ForeignKey(Plan, on_delete=models.CASCADE, related_name='user_plan')

然后使用以下内容：

queryset = User.objects.all() \
   .prefetch_related(
       Prefetch('user_plan__plan', 
          queryset=plan_queryset
       )
   )