Validate a unique DB Field

unique.php

    public function validate()
    {
        $result = parent::validate();
        $this->ensureUnique('Title', $result);

        return $result;
    }

    /**
     * @param string            $fieldName
     * @param \ValidationResult $validationResult
     *
     * @return \ValidationResult
     */
    protected function ensureUnique($fieldName, &$validationResult)
    {
        if ($this->isChanged($fieldName)) {
            $exists = myClass::get()->filter($fieldName, $this->getField($fieldName))->first();
            if ($exists) {
                $validationResult->error($fieldName.' must be unique among '. myClass::class);
            }
        }

        return $validationResult;
    }