Question

我有一个查询，目前它在shell中使用Django ORM正常工作：

>>> Place.objects.all()[0].images.filter(order=0)[0].filename
'y5IUMPyv.jpg'

但我不知道如何使用DRF的SerializerMethodField实现它。这是我在此期间使用的，直到我能弄明白：

class CardSerializer(serializers.Serializer):
    image = serializers.SerializerMethodField()

    @staticmethod
    def get_image(obj):
        for d in obj.images.all():
            if d.order == 0:
                return d.filename

这是＆＃34;理想的＆＃34;不起作用的方法，我不知道原因：

    @staticmethod
    def get_image(obj):
        return obj.images.filter(order=0)[0].filename

Traceback (most recent call last):
  File "/home/admin/env/lib/python3.4/site-packages/django/core/handlers/base.py", line 149, in get_response
    response = self.process_exception_by_middleware(e, request)
  File "/home/admin/env/lib/python3.4/site-packages/django/core/handlers/base.py", line 147, in get_response
    response = wrapped_callback(request, *callback_args, **callback_kwargs)
  File "/home/admin/env/lib/python3.4/site-packages/django/views/decorators/csrf.py", line 58, in wrapped_view
    return view_func(*args, **kwargs)
  File "/home/admin/env/lib/python3.4/site-packages/django/views/generic/base.py", line 68, in view
    return self.dispatch(request, *args, **kwargs)
  File "/home/admin/env/lib/python3.4/site-packages/rest_framework/views.py", line 466, in dispatch
    response = self.handle_exception(exc)
  File "/home/admin/env/lib/python3.4/site-packages/rest_framework/views.py", line 463, in dispatch
    response = handler(request, *args, **kwargs)
  File "/home/admin/env/lib/python3.4/site-packages/rest_framework/generics.py", line 201, in get
    return self.list(request, *args, **kwargs)
  File "/home/admin/env/lib/python3.4/site-packages/rest_framework/mixins.py", line 45, in list
    return self.get_paginated_response(serializer.data)
  File "/home/admin/env/lib/python3.4/site-packages/serpy/serializer.py", line 134, in data
    self._data = self.to_value(self.instance)
  File "/home/admin/env/lib/python3.4/site-packages/serpy/serializer.py", line 123, in to_value
    return [serialize(o, fields) for o in instance]
  File "/home/admin/env/lib/python3.4/site-packages/serpy/serializer.py", line 123, in <listcomp>
    return [serialize(o, fields) for o in instance]
  File "/home/admin/env/lib/python3.4/site-packages/serpy/serializer.py", line 107, in _serialize
    result = getter(self, instance)
  File "/home/admin/src/places/serializers.py", line 72, in get_image
    return obj.images.filter(order=0)[0].filename
  File "/home/admin/env/lib/python3.4/site-packages/django/db/models/query.py", line 297, in __getitem__
    return list(qs)[0]
IndexError: list index out of range

Answer 1

正如铁娘子@ anderson-lima指出的那样，这是你的数据而不是代码的问题。您没有order = 0的图像，并且如果不是最佳的话，您的第一个方法会处理这种情况。

NSLog

然而，在你的第二种方法中，你正在获取切片，但是获取一个不存在的对象。因此，执行，并反过来告诉我们你需要的只是一个尝试，除了块。

@staticmethod
    def get_image(obj):
        for d in obj.images.all():
            if d.order == 0:
                return d.filename

        # returns None here if an object with order = 0
        # does not exist in the database.

DRF如何优化此查询，而不是循环，如果是？

1 个答案: