Get a custom field object for this model
        field_name - Name of the custom field you want.

def get_custom_field(self, field_name):
        """ Get a custom field object for this model
        field_name - Name of the custom field you want.
        """
        content_type = ContentType.objects.get_for_model(self)
        return CustomField.objects.get(
            content_type=content_type, name=field_name)