Apply belongs_to_required_by_default to all models

Unless they state otherwise.

The new standard also changes the default behaviour of the Shoulda
matcher in Rspec. It now defaults to asserting that an association is
required. That needed some spec updates.

In one case, Spree::Product, I also had to update the model because the
presence validation was somehow not recognised by the Shoulda matcher.
The error message may change slightly but the outcome should be the
same.

app/models/spree/product.rb

@@ -34,8 +34,8 @@ module Spree
     searchable_scopes :active, :with_properties
 
     belongs_to :shipping_category, class_name: 'Spree::ShippingCategory'
-    belongs_to :supplier, class_name: 'Enterprise', touch: true
-    belongs_to :primary_taxon, class_name: 'Spree::Taxon', touch: true
+    belongs_to :supplier, class_name: 'Enterprise', optional: false, touch: true
+    belongs_to :primary_taxon, class_name: 'Spree::Taxon', optional: false, touch: true
 
     has_one :image, class_name: "Spree::Image", as: :viewable, dependent: :destroy
 
@@ -54,9 +54,6 @@ module Spree
     validates :name, presence: true
     validates :shipping_category, presence: true
 
-    validates :supplier, presence: true
-    validates :primary_taxon, presence: true
-
     validates :variant_unit, presence: true
     validates :unit_value, presence:
       { if: ->(p) { %w(weight volume).include?(p.variant_unit) && new_record? } }

config/application.rb

@@ -227,7 +227,6 @@ module Openfoodnetwork
     # https://guides.rubyonrails.org/configuring.html#results-of-config-load-defaults
     config.load_defaults 6.1
     config.action_view.form_with_generates_remote_forms = false
-    config.active_record.belongs_to_required_by_default = false
     config.active_record.cache_versioning = false
     config.active_record.has_many_inversing = false
     config.active_record.yaml_column_permitted_classes = [BigDecimal, Symbol]

spec/models/spree/adjustment_spec.rb

@@ -11,10 +11,10 @@ module Spree
       it { is_expected.to have_one(:metadata) }
       it { is_expected.to have_many(:adjustments) }
 
-      it { is_expected.to belong_to(:adjustable) }
-      it { is_expected.to belong_to(:originator) }
-      it { is_expected.to belong_to(:order) }
-      it { is_expected.to belong_to(:tax_category) }
+      it { is_expected.to belong_to(:adjustable).optional }
+      it { is_expected.to belong_to(:originator).optional }
+      it { is_expected.to belong_to(:order).optional }
+      it { is_expected.to belong_to(:tax_category).optional }
     end
 
     describe "scopes" do

spec/models/spree/product_spec.rb

@@ -154,8 +154,8 @@ module Spree
     end
 
     describe "associations" do
-      it { is_expected.to belong_to(:supplier) }
-      it { is_expected.to belong_to(:primary_taxon) }
+      it { is_expected.to belong_to(:supplier).required }
+      it { is_expected.to belong_to(:primary_taxon).required }
     end
 
     describe "validations and defaults" do

spec/models/subscription_spec.rb

@@ -4,13 +4,13 @@ require 'spec_helper'
 
 describe Subscription, type: :model do
   describe "associations" do
-    it { expect(subject).to belong_to(:shop) }
-    it { expect(subject).to belong_to(:customer) }
-    it { expect(subject).to belong_to(:schedule) }
-    it { expect(subject).to belong_to(:shipping_method) }
-    it { expect(subject).to belong_to(:payment_method) }
-    it { expect(subject).to belong_to(:ship_address) }
-    it { expect(subject).to belong_to(:bill_address) }
+    it { expect(subject).to belong_to(:shop).optional }
+    it { expect(subject).to belong_to(:customer).optional }
+    it { expect(subject).to belong_to(:schedule).optional }
+    it { expect(subject).to belong_to(:shipping_method).optional }
+    it { expect(subject).to belong_to(:payment_method).optional }
+    it { expect(subject).to belong_to(:ship_address).optional }
+    it { expect(subject).to belong_to(:bill_address).optional }
     it { expect(subject).to have_many(:subscription_line_items) }
     it { expect(subject).to have_many(:order_cycles) }
     it { expect(subject).to have_many(:proxy_orders) }