Re: DoesNotExist behavior in db.models query.py

The difference between .filter() and .get() is definitely 'by-design'.

When you filter through a list of objects, you could end up with zero,
one, or many objects returned.

When you call .get(), you are basically saying "I want to get exactly
*one* record". If the record is not found, it is considered an error.

One additional difference is that .filter() returns a list of zero or
more objects whereas .get() returns the object you requested or it
throws an error.

You can catch the error calling .get() like so:

try:
Person.objects.get(pk=123)
except Person.DoesNotExist:
print("That person did not exist")


-A

On Mon, Oct 23, 2017 at 3:14 PM, <steve@fitcode.com> wrote:
> Replying to myself here: as is often the case, the problem lies with my own
> source code.
> I was calling
> MODELNAME.objects.get(modelfieldname1='foo',modelfieldname2='bar')
>
>
> which raises the 500 when no such record exists.
>
> Turns out that calling
> MODELNAME.objects.filter(modelfieldname1='foo',modelfieldname2='bar')
>
>
> behaves politely when finding no matching record and returns a 0 length
> QuerySet array. So with calling filter I can then just test on length of
> result and make my behavior omit a 400 and some helpful information to
> client side about the nature of the 400 error.
>
> Separately, this does raise the question of should get() and filter() behave
> similarly or is there a reason that filter() allows empty result sets but
> get() does not?
>
> Thanks,
> Steve
>
> On Monday, October 23, 2017 at 2:49:28 PM UTC-7, st...@fitcode.com wrote:
>>
>> I have been tripping over the following exception
>> DoesNotExist: INSERT_YOUR_MODEL_NAME_HERE matching query does not exist.
>>
>> The error is source in django 1.11.2 in db.models query.py file, in the
>> get(self, *args, **kwargs) function, line 378 to be exact
>> ...
>> raise self.model.DoesNotExist(...)
>> ...
>> This throws a 500 error and I would like my code not to throw a 500 error
>> in this situation, rather a customized message more along a 4xx error e.g.
>> {"message": "record matching identifier A and identifier B not found in
>> database"}
>> rather than a 500 error.
>>
>> It seems to me that my only choice right now is to subclass QuerySet and
>> write my own version of the get(self, *args, **kwargs) method. Is that my
>> only choice or is there a django parameter I can set so that 0 matching
>> results does not return a 500?
>>
>> Or do I need to intercept the 500 error somewhere else.
>>
>> Sorry, am a bit of a newbie to django internals, all help and advice is
>> appreciated.
>>
>> Thanks,
>> Steve
>
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