DRF如何优化这个查询，而不是循环if？

Question

我心里有一个查询，目前它按预期工作与Django的ORM壳：

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

但我不知道如何与DRF的SerializerMethodField实现它。这是我在此期间使用什么，直到我自己看着办吧：

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“理想”的方法还是不行，我不知道为什么：

@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

由于铁娘子@安德森 - 利马指出，这是与您的数据，而不是你的代码中的问题。您没有order = 0的图像，如果不是最佳，您的第一个方法可以正确处理这种情况。

@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. 

但是，在第二种方法中，您正在切片，但是获取不存在的对象。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): 
    try: 
     return obj.images.filter(order=0)[0].filename 
    except IndexError: 
     return None