+ # Return nil if current user is not allowed to see the list of
+ # writers. Otherwise, return a list of user_ and group_uuids with
+ # write permission. (If not returning nil, current_user is always in
+ # the list because can_manage permission is needed to see the list
+ # of writers.)
+ def writable_by
+ unless (owner_uuid == current_user.uuid or
+ current_user.is_admin or
+ current_user.groups_i_can(:manage).index(owner_uuid))
+ return nil
+ end
+ [owner_uuid, current_user.uuid] + permissions.collect do |p|
+ if ['can_write', 'can_manage'].index p.name
+ p.tail_uuid
+ end
+ end.compact.uniq
+ end
+