code: public function validateAttribute($model, $attribute)
      {
          /* @var $targetClass ActiveRecordInterface */
          $targetClass = $this->getTargetClass($model);
          $targetAttribute = $this->targetAttribute === null ? $attribute : $this->targetAttribute;
          $rawConditions = $this->prepareConditions($targetAttribute, $model, $attribute);
          $conditions = [$this->targetAttributeJunction === 'or' ? 'or' : 'and'];
  
          foreach ($rawConditions as $key => $value) {
              if (is_array($value)) {
                  $this->addError($model, $attribute, Yii::t('yii', '{attribute} is invalid.'));
                  return;
              }
              $conditions[] = [$key => $value];
          }
  
          $db = $targetClass::getDb();
  
          $modelExists = false;
  
          if ($this->forceMasterDb && method_exists($db, 'useMaster')) {
              $db->useMaster(function () use ($targetClass, $conditions, $model, &$modelExists) {
                  $modelExists = $this->modelExists($targetClass, $conditions, $model);
              });
          } else {
              $modelExists = $this->modelExists($targetClass, $conditions, $model);
          }
  
          if ($modelExists) {
              if (is_array($targetAttribute) && count($targetAttribute) > 1) {
                  $this->addComboNotUniqueError($model, $attribute);
              } else {
                  $this->addError($model, $attribute, $this->message);
              }
          }
      }
docs: {@inheritdoc}