Rails Ajax调用返回500

Question

I have got a ajax call which deletes some data from database. The data is deleted properly, but The action returns a failure ...

code looks like this:

def remove_user_from_group
  @usergroup = UserGroup.find params[:usergroup_id]
  @user = User.find params[:user_id]
  @usergroup.users.delete(@user)
  if @usergroup.save
    flash[:notice] = t("flash.deleted")
    render :add_user_to_group
  else
    flash[:notice] = t("flash.login_failed_nouser")
    render :add_user_to_group
  end
end

Whats going wrong? I do exactly the same for adding data and it works fine.