rails scope model without relation

Question

I have such model

class Ads::Posting < ActiveRecord:Base
  has_one :child, class_name: 'Ads::Posting', foreign_key: :posting_id
  belongs_to :parent, class_name: 'Ads::Posting', foreign_key: :posting_id
end

I need to write scope which gets all postings without child. Any ideas how to do that?

Answer 1

that model doesnt make sense!

a children is automatically a parent.

what exactly do you want to make? a tree?

then you need to add to migration

t.integer :parent_id, null: false

and in your model

has_one :child, class_name: 'Ads::Posting', foreign_key: :parent_id

for making it deeper, have a look at

https://github.com/stefankroes/ancestry

maybe thats helping you.

Answer 2

I believe you can do this using a LEFT JOIN and a restriction on the where clause.

scope :my_scope, 
  joins("LEFT JOIN postings ON postings.ad_id = ads.id").where("postings.id IS NULL")