DRF How to optimize this query, instead of looping if's? - django

Question

I have a query in mind, currently it works as expected in shell with the Django ORM:
>>> Place.objects.all()[0].images.filter(order=0)[0].filename
'y5IUMPyv.jpg'
But I don't know how to implement it with DRF's SerializerMethodField. This is what I'm using in the meantime until I can figure it out:
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
This is the "ideal" method that doesn't work, and I don't know why:
#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

As the iron maiden #anderson-lima has pointed out, this is a problem with your data rather than your code. You do not have an image with order = 0 and your first method handles that situation correct if not optimally.
#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.
However in your second approach you are taking a slice but fetching an object that does not exist. Hence the execption, and which in turn tells us that what you need is a just a try except block.
#staticmethod
def get_image(obj):
try:
return obj.images.filter(order=0)[0].filename
except IndexError:
return None