delete in a relation doesn't call sweeper

Summary

• @users_friend.user.friends.delete(@users_friend.friend) # Dont call the sweeper observer
• @users_friend.destroy # Call the sweeper
• @users_friend.user.friends << @users_friend.friend # Call the sweeper

In these situation, destroy and destroy2 do similary things.
* One do: @users_friend.user.friends.delete(@users_friend.friend) * And the other: @users_friend.destroy

Only the second one call the observer. I can understand these, and imagine why happens. But if these doesn't execute as expected, something like:

@users_friend.user.friends << @users_friend.friend

Must don't run too.

I put the important code here, and add a sample app as attachment.

class User < ActiveRecord::Base
  has_many :users_friends
  has_many :friends, :through => :users_friends
end

class UsersFriend < ActiveRecord::Base
  belongs_to :user
  belongs_to :friend, :class_name => "User", :foreign_key => "friend_id"
end

class UsersFriendSweeper < ActionController::Caching::Sweeper
  observe UsersFriend
  def after_destroy(record)
    puts "after_destroy"
    debugger
  end
  def before_destroy(record)
    puts "before_destroy"
    debugger
  end
end

class UsersFriendsController < ApplicationController
  cache_sweeper :users_friend_sweeper

...

  # POST /users_friends
  # POST /users_friends.xml
  def create
    @users_friend = UsersFriend.new(params[:users_friend])

    @users_friend.user.friends << @users_friend.friend #These call the sweeper before_save
    respond_to do |format|
        flash[:notice] = 'UsersFriend was successfully created.'
        format.html { redirect_to(@users_friend.user) }
        format.xml  { render :xml => @users_friend, :status => :created, :location => @users_friend }
        format.js
    end
  end

  def destroy
    @users_friend = UsersFriend.find(params[:id])
    @users_friend.user.friends.delete(@users_friend.friend) #Dont call the sweeper

    respond_to do |format|
      format.html { redirect_to(users_friends_url) }
      format.xml  { head :ok }
      format.js   { render :action => "create"}
    end
  end

  def destroy2
    @users_friend = UsersFriend.find(params[:id])
    @users_friend.destroy #These call the sweeper before_destroy

    respond_to do |format|
      format.html { redirect_to(users_friends_url) }
      format.xml  { head :ok }
      format.js   { render :action => "create"}
    end
  end
...
end