PartyList attributes and the plugin event pipeline

This should be a quick post about a subtlety with the plugin event pipeline. I recently wrote a plugin that could modify the data that's submitted when updating an activity record. This should have been a straightforward plugin on the Pre event that modified the Target InputParameter, and it all worked fine, except for partylist fields (such as the resources field on the serviceappointment entity, or optionalattendees on the appointment entity). Essentially, any changes I made to these fields in the plugin were ignored.

Fortunately, there is a solution, and it depends on the stage that you register the plugin on. If you register on stage=20 (i.e. within the transaction), your changes are ignored. However, change the registration to stage=10 (before the transaction), then it does work. There's no documentation on this, but I expect it is due to how the partylist data is saved. This data is written to the activityparty table in the database, and I expect that the SQL for this is already fixed at the start of the transaction, and hence is unaffected by changes in the plugin code.