Django form validation error with incomplete fields but complete instance
def update_profile(request, username):
    user = Profile.objects.get(user__username=username)

    # update subset of profile, eg value_to_company is set in request.POST
    # but user (or an arbitrary number of other attributes) is not
    profile = ProfileForm(request.POST, instance=user)

    if not profile.is_valid():
        print profile.errors  #user is required

class Profile(models.Model):
    user = models.ForeignKey(auth.models.User)
    value_to_company = models.IntegerField()

class ValueField(forms.Field):
    def to_python(self, value):
        vals = {'high': 0,
                'mid': 1}
        return vals[value]

class ProfileForm(forms.ModelForm):
    value_to_company = ValueField()

    class Meta:
        model = Profile

# replaces `if not profile.is_valid()` above:
errors = []
for field in request.POST.iterkeys():
    if field in profile.fields:
        profile.fields[field].to_python()
        if not profile.fields['field'].clean():
            errors.append #something

class BaseModelForm(forms.ModelForm):
    """
    Subclass of `forms.ModelForm` which makes sure that the instance values are present in the form
    data, so you don't have to send all old values for the form to actually validate. Django does not
    do this on its own.
    """

    def merge_from_instance(self):
        # Internals
        self.data._mutable = True

        fun = lambda v: v not in self.data.keys()
        for field in filter(fun, self.fields.keys()):
            if field not in self.initial:
                continue
            value = self.initial.get(field)
            if isinstance(self.instance._meta.get_field(field), related.ManyToManyField):
                self.data.setlist(field, value)
            else:
                self.data[field] = value