framework/validators/UniqueValidator.php
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, $this->message);
            }
        }
    }
public function validateAttribute($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);
            }
        }
    }
{@inheritdoc}