Add permanent filter on the collection

        :param field_name: name of the field to filter on
        :type field_name: str
        :param field_value: value to filter on

        :rtype: Collection

def filter(self, field_name, field_value):
        """Add permanent filter on the collection

        :param field_name: name of the field to filter on
        :type field_name: str
        :param field_value: value to filter on

        :rtype: Collection
        """
        self.filters.append((field_name, field_value))
        return self