How do you reference only the persisted records in an active record association

Question

In the edit method of many controllers you initialize a new object and edit existing objects

class MagazinesController < ApplicationController
   def edit
      @magazine = Magazine.find(params[:magazine_id])
      @page = Page.find(params[:id])
      @new_page = @magazine.pages.new
   end
end

However in a view you will often want to cycle through the persisted objects and treat the new object separately

# magazines#edit
%h4 Existing pages
- @magazine.pages.each do |page|
  %p= link_to page, page.title

The problem

...is that the pages association contains both existing (persisted) pages but also the new page which we made via @new_page = @magazine.pages.new.

It's easy to deal with this however it's ugly

%h4 Existing pages
- @magazine.pages.each do |page|
  - if page.persisted?
    %p= link_to page, page.title

I would like to use some assocition method to select only those pages which are persisted:

%h4 Existing pages
- @magazine.pages.persisted.each do |page|
  %p= link_to page, page.title

Is there any way of doing this?

Answer 1

You can create in your Page model a persisted scope: scope :persisted, -> { where "id IS NOT NULL" }, which avoids iterating on each associated page to check whether it's a new record or not.

Answer 2

Another cleaner syntax, using ActiveRecord where.not and still get back an ActiveRecord collection:

- @magazine.pages.where.not(id: nil).each do |page|
    ...