I'm trying to create an object and adding an existing object to a "has_many through" association, but after saving my object the reference to my newly created object is set to nil in the join model.

To be specific, I'm creating a Notification object and adding a pre-existing Member object to the Notification.members association. I'm using nested resources and I'm invoking the notification controller's new function using the following relative URL: /members/1/notifications/new

After filling out the form and submitting, the create function is called, and from wha开发者_如何学运维t I understand from the Rails Associations guide, section 4.3.3 "When are Objects Saved?", the members associations should be created in the database when the new notification object is saved:

"If the parent object (the one declaring the has_many association) is unsaved (that is, new_record? returns true) then the child objects are not saved when they are added. All unsaved members of the association will automatically be saved when the parent is saved."

After creating the notification object, the following record was created in the database:

select id, notification_id, notifiable_type, notifiable_id from deliveries; 
1|<NULL>|Member|1 

I worked around the problem by saving the notification object before adding the member object to the association. At first this seemed to be an ok solution for now, but I soon discovered that this has it's downsides. I don't want to save the notification without it's member association since I then have to write workarounds for my callbacks so that they don't start performing tasks on the not yet valid notification object.

What am I doing wrong here? All tips are appreciated. :D

Models

class Notification < ActiveRecord::Base
  has_many :deliveries, :as => :notifiable
  has_many :members, :through => :deliveries, :source => :notifiable, :source_type => "Member"
  has_many :groups, :through => :deliveries, :source => :notifiable, :source_type => "Group"
end

class Member < ActiveRecord::Base
  has_many :deliveries, :as => :notifiable
  has_many :notifications, :through => :deliveries
end

class Delivery < ActiveRecord::Base
  belongs_to :notification
  belongs_to :notifiable, :polymorphic => true
end

# Group is not really relevant in this example.
class Group < ActiveRecord::Base 
  has_many :deliveries, :as => :notifiable
  has_many :notifications, :through => :deliveries
end

Controller

class NotificationsController < ApplicationController
  def create
    @notification = Notification.new(params[:notification])
    @member = Member.find(params[:member_id])
    @notification.members << @member

    respond_to do |format|
      if @notification.save
        ...
      end
    end
  end
end

After posting a bug report, I got som help from one of the Rails gurus. In short, it's impossible to get this working the way I thought.

I decided to proceed with slightly more controller code, seems to be working just fine:

def create
    @notification = Notification.new(params[:notification])
    @member = Member.find(params[:member_id])

    respond_to do |format|
      if @notification.save
        @member.notifications << @notification
        @member.save
        ...