FIX: Watching First Post in groups was working incorrectly

app/services/post_alerter.rb

@@ -120,7 +120,7 @@ class PostAlerter
                             .where(notification_level: TagUser.notification_levels[:watching_first_post])
                             .pluck(:user_id)
 
-        group_ids = post.user.groups.pluck(:id)
+        group_ids = topic.allowed_groups.pluck(:group_id)
         group_watchers = GroupUser.where(group_id: group_ids,
                                          notification_level: GroupUser.notification_levels[:watching_first_post])
                                   .pluck(:user_id)

spec/services/post_alerter_spec.rb

@@ -363,6 +363,7 @@ describe PostAlerter do
     it "notifies the user who is following the first post group" do
       GroupUser.create(group_id: group.id, user_id: user.id)
       GroupUser.create(group_id: group.id, user_id: post.user.id)
+      topic.topic_allowed_groups.create(group_id: group.id)
 
       level = GroupUser.notification_levels[:watching_first_post]
       GroupUser.where(user_id: user.id, group_id: group.id).update_all(notification_level: level)