Hi,
Thank you very much for your help.
I used your form validation method.
The results is weird.
I tested it with a value lower than the previous value. But didn't see
any error message in my browser.
I stepped through the code using pdb. I could see that
forms.ValidationError("The amount in field yourfieldname may only
increase.")
was raised.
When I looked at the view that processes the form, I could also see
that form.is_valid() returned False.
Yet the next lines executed were:
variables = RequestContext(request, {'form': form})
return render_to_response('template.html', variables)
Those are the lines to be executed if there is no POST data.
I thought if form.is_valid() returns False, Django would send a list
of the errors to browser. But that's not what happened here.
Any ideas why?
On Dec 6, 11:20 pm, Shawn Milochik <sh...@milochik.com> wrote:
> On Dec 6, 2009, at 11:01 PM, Continuation wrote:
>
> > What do you mean by the __init__ of the model?
>
> Every models.Model object has an __init__ function, like any Python class.
> You need to override it.
>
> Add this into your model (in your models.py):
>
> def __init__(self, *args, **kwargs):
>
> #make sure you call the default __init__, or stuff breaks.
> super(YourModelName, self).__init__(*args, **kwargs)
>
> #so we can tell if the new value for the field
> #is increased upon save
> self.old_value = self.field_name
>
> Then, you override the save function for the same model:
>
> def save(self):
>
> if self.old_value < self.fieldname:
> raise ValueError('The field x can only increase.')
>
> #go through with the standard 'save' stuff
> #that is defined in models.Model class.
> super(YourModelName, self).save()
>
> This way, your model will not allow itself to be saved if that value is
> lowered. This doesn't directly answer your question, which was about form
> validation. I don't know how to do it elegantly, so I hope someone else will
> chime in. However, one thing you can do is to override the "clean" function
> of your form for that particular field so that it pulls back an instance of
> the old object and checks its own cleaned_data value against the value stored
> in the database. Something like this:
>
> #within your forms.ModelForm for the model in question
>
> def clean_yourfieldname(self):
>
> yourfieldname = self.cleaned_data.get('yourfieldname', 0)
> old_instance = YourModel.objects.get(pk = self.instance.pk)
>
> if yourfieldname < old_instance.yourfieldname:
> raise forms.ValidationError("The amount in field yourfieldname
> may only increase.")
>
> return yourfieldname
>
> To be fair, if you use this in your form validation, you don't really need
> the stuff above in your model. However, if that is your business logic, then
> it's a good idea to have it there in any case.
>
> Shawn
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