DoesNotExist ambiguous exception when accessed via foreign key

[simon@…]

Hi.

When a Foo.DoesNotExist exception is raised, the debug page only prints 'DoesNotExist', without showing which model it belongs to.

Is there some way we can fix this? I would like to see as much information as possible.

Thanks.
Simon

[anonymous]

Below the exception type is the exception value which will usually say something like

FooBar matching query does not exist

Then below that is a full traceback which should point you to which piece of code which made the erroneous query.

[Jannis Leidel]

Yeah, this needs more information. What is the expected output?

[Simon Williams]

When accessed via a query, this does indeed give the model name in the message (e.g. 'FooBar matching query does not exist').

However, when accessing a foreign key which either has an invalid value or is null, the exception is raised without any message:

.../django/db/models/fields/related.py:301 in __get__
raise self.field.rel.to.DoesNotExist

This causes the debug page just to show 'DoesNotExist' and 'No exception supplied'.

Since Model.DoesNotExist is created dynamically when the model is loaded, surely it would be possible to the name of the model as the default message or similar?

Thanks.

[JordanPowell]

Any idea why this is DDN? I can't see any problem with making the exception more informative. It looks like someone just forgot to instantiate it.

I'm happy to put together a patch and tests for this, if someone agrees that there's no Design Decision Needed.

I'd probably just change the line to:

raise self.field.rel.to.DoesNotExist(
    "This %s has no %s (the %s is null)" %
    (self.field.model._meta.object_name, self.field.name,
    self.field.rel.to._meta.object_name))

so it reads:

This Model has no field (the RelatedModel is null).

Any comments on the wording?

[Simon Charette]

Accepting the ticket as an improvement.

raise self.field.rel.to.DoesNotExist(
    "%s has no %s." % (self.field.model.__name__, self.field.name)
)

Model has no field. sounds better to me but I'd also like comments on the wording.

I removed the null part since this exception can also be raised if the field as an invalid value as pointed out in comment:3.

This needs testing and an addition to the release note.

[JordanPowell]

I'll put something together tomorrow if no one beats me to it.

[JordanPowell]

Just added a patch for this - let me know if anything needs changing and if I should issue a pull request.

[Simon Charette]

Instead of testing for the exception message being not empty you should use assertRaisesMessage to make sure the correct message is raised.

[JordanPowell]

Ok cool - how's that? I wasn't sure if I should tie the test so closely to the message, thanks for clarifying.

[Simon Charette]

Apart from the missing point in the release note the patch looks RFC. Will commit soon with this minor change.

[Tim Graham]

In reviewing this, I found another instance of raise DoesNotExist without a message. Updated in the following pull request: ​https://github.com/django/django/pull/1212

(moving back to accepted to get another review)

[Aymeric Augustin]

Looks good!

[Tim Graham <timograham@…>]

In 6f8627dd7fd482007694dd1e4a99f9e207e6d8c2:

Fixed #17582 - Added message to DoesNotExist exceptions.

Thanks simon@ for the suggestion and JordanPowell
for the initial patch.