A logging device adapted to be attached to a pen drug delivery device (210) and capture a property value related to the dose amount of drug expelled from the drug delivery device during an expelling event, the logging device comprising a bore in which a snap lock with a flexible wire portion is arranged circumferentially in the bore, the wire being adapted to be moved laterally by a projection on the pen device and subsequently snap inwardly when the pen device is inserted axially in the bore. In this way an axial snap lock is provided between the logging device and the drug delivery device.
