DRF如何優化這個查詢，而不是循環if？

Question

我心裏有一個查詢，目前它按預期工作與Django的ORM殼：

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

但我不知道如何與DRF的SerializerMethodField實現它。這是我在此期間使用什麼，直到我自己看着辦吧：

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 

這是「理想」的方法還是不行，我不知道爲什麼：

@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

由於鐵娘子@安德森 - 利馬指出，這是與您的數據，而不是你的代碼中的問題。您沒有order = 0的圖像，如果不是最佳，您的第一個方法可以正確處理這種情況。

@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，在第二種方法中，您正在切片，但是獲取不存在的對象。因此，執行，並反過來告訴我們，你需要的只是一個嘗試，除了塊。

@staticmethod 
def get_image(obj): 
    try: 
     return obj.images.filter(order=0)[0].filename 
    except IndexError: 
     return None