An assembly is presented having a reservoir holder, a dose setting mechanism, and flexible fastener configured to fasten the reservoir holder to the dose setting mechanism, wherein the flexible fastener is provided at one of the reservoir holder and the dose setting mechanism. The flexible fastener is configured to releasably fasten the reservoir holder and the dose setting mechanism to each other. Moreover, said flexible fastener is arranged on a retention ring provided at the one of the reservoir holder and the dose setting mechanism. Furthermore, a drug delivery device with such an assembly is disclosed.