public function field($fieldName, $fieldValue = '', $fieldBoost = null) {
        if(empty($fieldName)) {
            throw new \InvalidArgumentException('Please provide a fieldname');
        }
        $field = array(
			'name'=> $fieldName,
			'value' => $fieldValue
        );
        if(!empty($fieldBoost)) {
            $field['boost'] = $fieldBoost;
        }
        $this->fields[] = $field;
        return $this;
    }