Handling unique_together errors in Django forms

forms.py

class MyModelForm(ModelForm):
    class Meta:
        model = MyModel
        fields = ('field2', 'field3', 'field4', 'field5')

    def clean(self):
        """
        Clean MyModel
        """
        # Get the cleaned data
        cleaned_data = self.cleaned_data

        # Find the unique_together fields
        field1 = cleaned_data.get('instance')
        field2 = cleaned_data.get('data_file')
        field3 = cleaned_data.get('column')
        field4 = cleaned_data.get('key')

        objects = MyModel.objects.filter( field1 = field1,
                                          field2 = field2,
                                          field3 = field3,
                                          field4 = field4)
        if len(objects) > 0:
            msg = u"This row is not unique"
            self._errors["field1"] = self.error_class([msg])
            del cleaned_data["field1"]
            raise ValidationError(msg)

        return cleaned_data

models.py

from django import forms

class MyModel(models.Model):
    class Meta:
        unique_together = ('field1', 'field2', 'field3', 'field4')

    field1    = models.ForeignKey(MyModel)
    field2    = models.CharField(null = False, max_length = 30)
    field3    = models.CharField(null = False, max_length = 30)
    field4    = models.CharField(null = False, max_length = 30)
    field5    = models.CharField(null = False, max_length = 80)

views.py

myform.is_valid()