public function addAttributeValue(string $attribute, string $value = ''): self
    {
        // Retrieve the current values of the attribute
        $values = $this->getAttribute($attribute);

        // Merge the values
        if (! in_array($value, $values)) {
            $values[] = $value;
        }

        // Store the new values
        $this->setAttribute($attribute, $values);

        return $this;
    }