Get a value for a specified custom field
        field_name - Name of the custom field you want.

def get_custom_value(self, field_name):
        """ Get a value for a specified custom field
        field_name - Name of the custom field you want.
        """
        custom_field = self.get_custom_field(field_name)
        return CustomFieldValue.objects.get_or_create(
            field=custom_field, object_id=self.id)[0].value