Validating ModelChoiceField in Django forms

Question

I'm trying to validate a form containing a ModelChoiceField:

forms.py:

from django import forms

from modelchoicetest.models import SomeObject

class SomeObjectAddForm(forms.ModelForm):
    class Meta:
        model = SomeObject

models.py:

from django.db import models

class SomeChoice(models.Model):
    name = models.CharField(max_length=16)

    def __unicode__(self):
        return self.name

class SomeObject(models.Model):
    choice = models.ForeignKey(SomeChoice)

views.py:

from django.shortcuts import render_to_response
from django.template import RequestContext
from django.http import HttpResponseRedirect
from django.core.urlresolvers import reverse

from forms import SomeObjectAddForm

def add(request):
    if request.method == 'POST':
        form = SomeObjectAddForm(request.POST)
        if form.is_valid():
            form.save()
            return HttpResponseRedirect(reverse('modelchoicetest_add'))
    else:
        form = SomeObjectAddForm()

    return render_to_response('modelchoicetest/index.html',
                              {'form': form},
                              context_instance=RequestContext(request))

When it is used in normal circumstances, everything goes just fine. But I'd like to protect the form from the invalid input. It's pretty obvious that I must get forms.ValidationError when I put invalid value in this field, isn't it? But if I try to submit a form with a value 'invalid' in 'somechoice' field, I get

ValueError: invalid literal for int() with base 10: 'invalid'

and not the expected forms.ValidationError. What should I do? I tried to place a def clean_somechoice(self) to check this field but that didn't work: ValueError happens before it comes to clean_somechoice()

Plus I don't think this is a good solution, there must be something more simple but I just missed that.

here's the full traceback:

Traceback:
File "/usr/local/lib/python2.6/dist-packages/django/core/handlers/base.py" in get_response
  101.                     response = callback(request, *callback_args, **callback_kwargs)
File "/home/andrey/public_html/example/modelchoicetest/views.py" in add
  11.         if form.is_valid():
File "/usr/local/lib/python2.6/dist-packages/django/forms/forms.py" in is_valid
  120.         return self.is_bound and not bool(self.errors)
File "/usr/local/lib/python2.6/dist-packages/django/forms/forms.py" in _get_errors
  111.             self.full_clean()
File "/usr/local/lib/python2.6/dist-packages/django/forms/forms.py" in full_clean
  276.                     value = field.clean(value)
File "/usr/local/lib/python2.6/dist-packages/django/forms/fields.py" in clean
  154.         value = self.to_python(value)
File "/usr/local/lib/python2.6/dist-packages/django/forms/models.py" in to_python
  911.             value = self.queryset.get(**{key: value})
File "/usr/local/lib/python2.6/dist-packages/django/db/models/query.py" in get
  330.         clone = self.filter(*args, **kwargs)
File "/usr/local/lib/python2.6/dist-packages/django/db/models/query.py" in filter
  536.         return self._filter_or_exclude(False, *args, **kwargs)
File "/usr/local/lib/python2.6/dist-packages/django/db/models/query.py" in _filter_or_exclude
  554.             clone.query.add_q(Q(*args, **kwargs))
File "/usr/local/lib/python2.6/dist-packages/django/db/models/sql/query.py" in add_q
  1109.                             can_reuse=used_aliases)
File "/usr/local/lib/python2.6/dist-packages/django/db/models/sql/query.py" in add_filter
  1048.                 connector)
File "/usr/local/lib/python2.6/dist-packages/django/db/models/sql/where.py" in add
  66.             value = obj.prepare(lookup_type, value)
File "/usr/local/lib/python2.6/dist-packages/django/db/models/sql/where.py" in prepare
  267.             return self.field.get_prep_lookup(lookup_type, value)
File "/usr/local/lib/python2.6/dist-packages/django/db/models/fields/__init__.py" in get_prep_lookup
  314.             return self.get_prep_value(value)
File "/usr/local/lib/python2.6/dist-packages/django/db/models/fields/__init__.py" in get_prep_value
  496.         return int(value)

Exception Type: ValueError at /
Exception Value: invalid literal for int() with base 10: 'invalid'

Answer 1

It looks to me like the exception is being raised by the clean method of the actual ModelChoiceField object. Because it's a foreign key, Django is expecting an int , which would be representative of the pk for SomeChoice . How exactly are you passing invalid into the form?

RESPONSE TO COMMENT

If you really feel you need to catch this, you can try overriding the default ModelChoiceField by creating a new field called choice and pass in the to_field_name kwarg into the ModelChoiceField __init__ method. This way Django won't be filtering on pk, and won't raise that exception.

Personally I wouldn't use this solution. There is no need to accommodate user who are hacking your form.

Answer 2

This is known Django bug:

http://code.djangoproject.com/ticket/11716

And while this bug is not fixed, you can only handle ValueError manually.