#3148: Add getters and setters to model fields
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Reporter: [email protected] | Owner: Gulopine
Status: new | Milestone:
Component: Database layer (models, ORM) | Version: SVN
Resolution: | Keywords:
Stage: Accepted | Has_patch: 1
Needs_docs: 1 | Needs_tests: 0
Needs_better_patch: 1 |
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Comment (by jerf):
Using the model:
{{{
class GetSet(models.Model):
field = models.CharField(max_length=200)
@field.setter
def field_setter(self, value):
self._field = value + "_test_transform"
@field.getter
def field_getter(self):
return self._field
}}}
we obtain with the latest patch "django-3148-approach-via-
descriptors.diff" the following python REPL output:
{{{
Python 2.6.4 (r264:75706, Nov 8 2009, 19:34:30)
[GCC 4.3.3] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> from mysite.modeltest.models import GetSet
>>> example = GetSet(field = "test")
>>> example.field
'test_test_transform'
>>> example.save()
>>> example.id
1
>>> del example
>>> example = GetSet.objects.get(id = 1)
>>> example.field
u'test_test_transform_test_transform'
>>> example.save()
>>> del example
>>> example = GetSet.objects.get(id = 1)
>>> example.field
u'test_test_transform_test_transform_test_transform'
>>>
}}}
That is not shippable behavior. The setter must not apply during
construction from a database load. Loading and immediately saving an
object with no changes must be a no-op, in terms of the stored data.
Anything else is asking for disaster.
I find myself wondering if perhaps the traffic on this bug is telling us
that the interaction of properties when the values are being backed by a
database store is just too complicated a concept to be worthwhile. When
the setters and getters would apply is more subtle than it first appears,
and perhaps "simple enough that it works as it first appears" (i.e., "no
property behavior") is a better idea.
I am not being sarcastic or passive aggressive. I am seriously wondering
this. I now think the documentation would have to explain this in more
detail than I originally thought, which adds a chunk of complexity right
where Django's documentation needs it least, and I'm no longer convinced
that's worth it. (Plus, after reviewing the latest SVN code, I still do
not see a way to do this correctly without setting the flag during
construction like I did in my original patch, which was not an...
''appreciated'' addition to the model loading code.)
--
Ticket URL: <http://code.djangoproject.com/ticket/3148#comment:34>
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