On 7/11/07, Carl Karsten <[EMAIL PROTECTED]> wrote:
> Huh. I would have expected this case would have been provided for.
No, the raising of the exception is quite deliberate on Django's part
-- when you use a method which is expected to return one and only one
object, Django has only two options:
1. Return the one, and only one, object, or
2. Raise an exception saying it couldn't do that.
Any other option introduces ambiguity and guesswork, and we don't want that.
If it can't do what you've asked, Django will raise one of these exceptions:
* If there is no such object, you'll get a model-specific subclass of
ObjectDoesNotExist.
* If there's more than one such object (e.g., you used 'get()' and the
supplied parameters matched multiple objects) you'll get an
AssertionError.
Internally, the various database querying methods Django exposes will
do their best to catch any possible exceptions and raise one of the
above two instead. This means that you have a consistent interface to
your data -- if you ask for something that isn't possible, you know
exactly what you're going to get and you don't have to guess about
exception types or empty return values.
The only time you won't get an exception is if you do something in a
template which would raise an ObjectDoesNotExist -- because the
template system explicitly advertises silent failure on
method/attribute access, ObjectDoesNotExist is treated specially and
fails silently.
So, for example, if you had this model:
class Entry(models.Model):
title = models.CharField(maxlength=250)
body = models.TextField()
pub_date = models.DateTimeField()
and only had one object of that model, you'd see one behavior in pure Python:
>>> e = Entry.objects.get(pk=1)
>>> e.get_previous_by_pub_date()
...(ObjectDoesNotExist gets raised here)...
But in a template the exception gets quashed, which means you can do
things like:
{% if e.get_previous_by_pub_date %}
The previous entry is {{ e.get_previous_by_pub_date.title }}.
{% else %}
This is the first entry.
{% endif %}
and not have to worry in your template about whether that's going to
raise an exception.
--
"Bureaucrat Conrad, you are technically correct -- the best kind of correct."
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