Merged r15609 (#23278).

git-svn-id: http://svn.redmine.org/redmine/branches/3.2-stable@15638 e93f8b46-1217-0410-a6f0-8f06a7374b81

lib/plugins/acts_as_watchable/lib/acts_as_watchable.rb

@@ -40,12 +40,16 @@ module Redmine
 
         # Adds user as a watcher
         def add_watcher(user)
+          # Rails does not reset the has_many :through association
+          watcher_users.reset
           self.watchers << Watcher.new(:user => user)
         end
 
         # Removes user from the watchers list
         def remove_watcher(user)
           return nil unless user && user.is_a?(User)
+          # Rails does not reset the has_many :through association
+          watcher_users.reset
           watchers.where(:user_id => user.id).delete_all
         end