fixes #3303

app/controllers/favorite_groups_controller.rb

@@ -63,7 +63,7 @@ class FavoriteGroupsController < ApplicationController
     @favorite_group = FavoriteGroup.find(params[:id])
     check_privilege(@favorite_group)
     @post = Post.find(params[:post_id])
-    @favorite_group.add!(@post)
+    @favorite_group.add!(@post.id)
   end
 
 private

app/models/favorite_group.rb

@@ -176,6 +176,7 @@ class FavoriteGroup < ApplicationRecord
     return if contains?(post_id)
 
     clear_post_id_array
+    puts ">>> post_id=#{post_id} post_ids=#{post_ids.inspect}"
     update_attributes(:post_ids => add_number_to_string(post_id, post_ids))
   end