Specify an optional reference in your Rails model

Question

I have a Sponsors model and a Promo Codes model.

• A sponsor can have zero or more promo codes
• A promo code can have zero or one sponsors

Thus a promo code should have an optional reference to a sponsor, that is, a sponsor_id that may or may not have a value. I'm not sure how to set this up in Rails.

Here's what I have so far:

# app/models/sponsor.rb
class Sponsor < ActiveRecord::Base
  has_many :promo_codes  # Zero or more.
end

---

# app/models/promo_code.rb
class PromoCode < ActiveRecord::Base
  has_one :sponsor  # Zero or one.
end

---

# db/migrate/xxxxx_add_sponsor_reference_to_promo_codes.rb
# rails g migration AddSponsorReferenceToPromoCodes sponsor:references
# Running migration adds a sponsor_id field to promo_codes table.
class AddSponsorReferenceToPromoCodes < ActiveRecord::Migration
  def change
    add_reference :promo_codes, :sponsor, index: true
  end
end

Does this make sense? I'm under the impression that I have to use belongs_to in my Promo Codes model, but I have no basis for this, just that I've haven't seen a has_many with has_one example yet.

Answer 1

In Rails 5, belongs_to is defined as required by default. To make it optional use the 'optional' option :)

class User
  belongs_to :company, optional: true
end

Source: https://github.com/rails/rails/issues/18233