Method test

/**
 * Method to test the username for uniqueness.
 *
 * @param   \SimpleXMLElement  $element  The SimpleXMLElement object representing the `<field>` tag for the form field object.
 * @param   mixed              $value    The form field value to validate.
 * @param   string             $group    The field name group control value. This acts as an array container for the field.
 *                                       For example if the field has name="foo" and the group value is set to "bar" then the
 *                                       full field name would end up being "bar[foo]".
 * @param   Registry           $input    An optional Registry object with the entire data set to validate against the entire form.
 * @param   Form               $form     The form object for which the field is being tested.
 *
 * @return  boolean  True if the value is valid, false otherwise.
 *
 * @since   3.9.0
 */
public function test(\SimpleXMLElement $element, $value, $group = null, Registry $input = null, Form $form = null)
{
    $value = trim($value);
    $existsTable = (string) $element['exists_table'];
    $existsColumn = (string) $element['exists_column'];
    // We cannot validate without a table name
    if ($existsTable === '') {
        return true;
    }
    // Assume a default column name of `id`
    if ($existsColumn === '') {
        $existsColumn = 'id';
    }
    $db = Factory::getDbo();
    // Set and query the database.
    $exists = $db->setQuery($db->getQuery(true)->select('COUNT(*)')->from($db->quoteName($existsTable))->where($db->quoteName($existsColumn) . ' = ' . $db->quote($value)))->loadResult();
    return (int) $exists > 0;
}