gistfile1.rb

  def update
    respond_to do |format|
      format.js {
        if request.xhr?
          @user = User.find(params[:id])
          params[:user][:location_ids] ||= []
          @user.assign_attributes(safe_params)
          if @user.save
            @users = users_list
            render template: 'users/list'
          else
            render template: 'users/form'
          end
        end
      }
    end
  end
  
  private
  
  def safe_params
    params.require(:user).permit(:username, :first_name, :last_name, :active, location_ids: [])
  end