framework/validators/ExistValidator.php
private function valueExists($targetClass, $query, $value)
    {
        $db = $targetClass::getDb();
        $exists = false;

        if ($this->forceMasterDb && method_exists($db, 'useMaster')) {
            $db->useMaster(function ($db) use ($query, $value, &$exists) {
                $exists = $this->queryValueExists($query, $value);
            });
        } else {
            $exists = $this->queryValueExists($query, $value);
        }

        return $exists;
    }
private function valueExists($targetClass, $query, $value)
    {
        $db = $targetClass::getDb();
        $exists = false;

        if ($this->forceMasterDb && method_exists($db, 'useMaster')) {
            $db->useMaster(function ($db) use ($query, $value, &$exists) {
                $exists = $this->queryValueExists($query, $value);
            });
        } else {
            $exists = $this->queryValueExists($query, $value);
        }

        return $exists;
    }
Check whether value exists in target table

@param string $targetClass
@param QueryInterface $query
@param mixed $value the value want to be checked
@return bool