Upgrade to v0.6

First run php bin/console doctrine:migrations:diff to get a fresh migration file. Now search for code VARCHAR(255) NOT NULL within that file. Immediately after that line add the following line:

$this->addSql("UPDATE setono_sylius_cms__asset SET code=SUBSTR(REPLACE(path, '/', ''), 1, 32)");

This will make sure the newly added code field will be unique.