Post function : add the assignee to the watchers without erasing the previous list of watchers

[Franklin Cadre]

Hi everyone, 

I would like to add the current assignee to the list of watchers. 

I tried and failed with these 2 options: 

1/ Post function for each transition : copy value from other field but then, the previous watchers are not watching anymore

2/ Use a transitional User Picker (multiple users) custom field in which i would, for each transition, copy the current watchers and current assignee using an "Update Issue Custom Field" post function. 

I would then copy the value of this field in the watchers. 

I tried this but when i the transition is blocked in my kanban board as the field cannot be updated. 

Any clue? 

Thank you