diff --git a/backend/spec/features/admin/products/edit/images_spec.rb b/backend/spec/features/admin/products/edit/images_spec.rb
index 1ff20b4ab19..d885bacb37b 100644
--- a/backend/spec/features/admin/products/edit/images_spec.rb
+++ b/backend/spec/features/admin/products/edit/images_spec.rb
@@ -78,7 +78,7 @@
         end
 
         click_button "Update"
-        Spree::Image.first.attachment.blob.update(key: 11)
+        invalidate_attachment(Spree::Image.first.attachment)
         visit current_path
         expect(page).to have_xpath("//img[contains(@src, 'assets/noimage/mini')]")
       end
@@ -136,4 +136,12 @@
       expect(page).to have_css("thead th", count: 4)
     end
   end
+
+  def invalidate_attachment(attachment)
+    blob = attachment.blob
+    blob.variant_records.each do |variant_record|
+      variant_record.update(variation_digest: SecureRandom.uuid)
+    end
+    blob.update(key: SecureRandom.uuid)
+  end
 end
diff --git a/core/app/models/spree/order.rb b/core/app/models/spree/order.rb
index 4443f97f9e8..72f7bc57d80 100644
--- a/core/app/models/spree/order.rb
+++ b/core/app/models/spree/order.rb
@@ -490,7 +490,10 @@ def create_proposed_shipments
         raise CannotRebuildShipments.new(I18n.t('spree.cannot_rebuild_shipments_shipments_not_pending'))
       else
         shipments.destroy_all
-        self.shipments = Spree::Config.stock.coordinator_class.new(self).shipments
+        # TODO: We can use `self.shipments#=` instead of `#push` when
+        # https://github.com/rails/rails/issues/42102 is fixed and we deprecate
+        # Rails versions with the bug
+        shipments.push(*Spree::Config.stock.coordinator_class.new(self).shipments)
       end
     end
 
diff --git a/core/app/models/spree/product.rb b/core/app/models/spree/product.rb
index 6928b3f4af7..3a99482bced 100644
--- a/core/app/models/spree/product.rb
+++ b/core/app/models/spree/product.rb
@@ -45,7 +45,6 @@ class Product < Spree::Base
 
     has_many :variants,
       -> { where(is_master: false).order(:position) },
-      inverse_of: :product,
       class_name: 'Spree::Variant'
 
     has_many :variants_including_master,
diff --git a/core/app/models/spree/stock/simple_coordinator.rb b/core/app/models/spree/stock/simple_coordinator.rb
index 6b0fe0d5786..e7fcce67236 100644
--- a/core/app/models/spree/stock/simple_coordinator.rb
+++ b/core/app/models/spree/stock/simple_coordinator.rb
@@ -73,11 +73,16 @@ def build_shipments
         packages = split_packages(packages)
 
         # Turn the Stock::Packages into a Shipment with rates
-        packages.map do |package|
+        shipments = packages.map do |package|
           shipment = package.shipment = package.to_shipment
           shipment.shipping_rates = Spree::Config.stock.estimator_class.new.shipping_rates(package)
           shipment
         end
+
+        # Make sure we don't add the proposed shipments to the order
+        order.shipments = order.shipments - shipments
+
+        shipments
       end
 
       def split_packages(initial_packages)
diff --git a/core/app/models/spree/variant.rb b/core/app/models/spree/variant.rb
index 591239bc914..a51d408e4ef 100644
--- a/core/app/models/spree/variant.rb
+++ b/core/app/models/spree/variant.rb
@@ -30,7 +30,7 @@ class Variant < Spree::Base
     attr_writer :rebuild_vat_prices
     include Spree::DefaultPrice
 
-    belongs_to :product, -> { with_discarded }, touch: true, class_name: 'Spree::Product', inverse_of: :variants, optional: false
+    belongs_to :product, -> { with_discarded }, touch: true, class_name: 'Spree::Product', inverse_of: :variants_including_master, optional: false
     belongs_to :tax_category, class_name: 'Spree::TaxCategory', optional: true
 
     delegate :name, :description, :slug, :available_on, :discontinue_on, :discontinued?,
diff --git a/core/lib/spree/testing_support/dummy_app.rb b/core/lib/spree/testing_support/dummy_app.rb
index b96ce9652c6..104139fe21e 100644
--- a/core/lib/spree/testing_support/dummy_app.rb
+++ b/core/lib/spree/testing_support/dummy_app.rb
@@ -46,6 +46,7 @@ def self.setup(gem_root:, lib_name:, auto_migrate: true)
   end
 
   class Application < ::Rails::Application
+    config.load_defaults("#{Rails::VERSION::MAJOR}.#{Rails::VERSION::MINOR}")
     config.eager_load = false
     config.cache_classes = true
     config.cache_store = :memory_store
diff --git a/core/spec/lib/search/variant_spec.rb b/core/spec/lib/search/variant_spec.rb
index 5c778417bfb..8d210e1d81c 100644
--- a/core/spec/lib/search/variant_spec.rb
+++ b/core/spec/lib/search/variant_spec.rb
@@ -59,6 +59,7 @@ def refute_found(query_string, variant)
           described_class.new(variant.sku, scope: Spree::Variant.in_stock).results
         ).to include variant
 
+        variant.stock_items.reload # See https://github.com/rails/rails/issues/42094
         variant.stock_items.each { |si| si.set_count_on_hand(0) }
         expect(
           described_class.new(variant.sku, scope: Spree::Variant.in_stock).results
diff --git a/core/spec/lib/spree/core/testing_support/dummy_app_spec.rb b/core/spec/lib/spree/core/testing_support/dummy_app_spec.rb
new file mode 100644
index 00000000000..953947ffa5e
--- /dev/null
+++ b/core/spec/lib/spree/core/testing_support/dummy_app_spec.rb
@@ -0,0 +1,11 @@
+# frozen_string_literal: true
+
+require 'rails_helper'
+
+RSpec.describe DummyApp do
+  it 'loads default from the Rails version in use' do
+    expect(
+      DummyApp::Application.config.loaded_config_version
+    ).to eq("#{Rails::VERSION::MAJOR}.#{Rails::VERSION::MINOR}")
+  end
+end
diff --git a/core/spec/models/spree/adjustment_spec.rb b/core/spec/models/spree/adjustment_spec.rb
index 5720d053f38..b50af5ac879 100644
--- a/core/spec/models/spree/adjustment_spec.rb
+++ b/core/spec/models/spree/adjustment_spec.rb
@@ -4,7 +4,7 @@
 
 RSpec.describe Spree::Adjustment, type: :model do
   let!(:store) { create :store }
-  let(:order) { Spree::Order.new }
+  let(:order) { create :order }
   let(:line_item) { create :line_item, order: order }
 
   let(:adjustment) { Spree::Adjustment.create!(label: 'Adjustment', adjustable: order, order: order, amount: 5) }
@@ -43,7 +43,7 @@
   end
 
   context '#currency' do
-    let(:order) { Spree::Order.new currency: 'JPY' }
+    let(:order) { create :order, currency: 'JPY' }
 
     it 'returns the adjustables currency' do
       expect(adjustment.currency).to eq 'JPY'
@@ -67,7 +67,7 @@
     end
 
     context "with currency set to JPY" do
-      let(:order) { Spree::Order.new currency: 'JPY' }
+      let(:order) { create :order, currency: 'JPY' }
 
       context "when adjustable is set to an order" do
         it "displays in JPY" do
diff --git a/core/spec/models/spree/order_contents_spec.rb b/core/spec/models/spree/order_contents_spec.rb
index 9bfa2e8b744..dc18f2030e0 100644
--- a/core/spec/models/spree/order_contents_spec.rb
+++ b/core/spec/models/spree/order_contents_spec.rb
@@ -25,7 +25,7 @@
 
       it "ensure shipment calls update_amounts instead of order calling ensure_updated_shipments" do
         expect(subject.order).to_not receive(:ensure_updated_shipments)
-        expect(shipment).to receive(:update_amounts)
+        expect(shipment).to receive(:update_amounts).at_least(:once)
         subject.add(variant, 1, shipment: shipment)
       end
 
diff --git a/core/spec/models/spree/payment_spec.rb b/core/spec/models/spree/payment_spec.rb
index 2a5a2125425..e38a7a832ff 100644
--- a/core/spec/models/spree/payment_spec.rb
+++ b/core/spec/models/spree/payment_spec.rb
@@ -8,7 +8,7 @@
   let(:refund_reason) { create(:refund_reason) }
 
   let(:gateway) do
-    gateway = Spree::PaymentMethod::BogusCreditCard.new(active: true, name: 'Bogus gateway')
+    gateway = Spree::PaymentMethod::BogusCreditCard.create!(active: true, name: 'Bogus gateway')
     allow(gateway).to receive_messages(source_required?: true)
     gateway
   end
diff --git a/core/spec/models/spree/product_spec.rb b/core/spec/models/spree/product_spec.rb
index 6b66a753b4b..cac6c32b703 100644
--- a/core/spec/models/spree/product_spec.rb
+++ b/core/spec/models/spree/product_spec.rb
@@ -129,7 +129,7 @@ class Extension < Spree::Base
         it "should set deleted_at value" do
           product.discard
           expect(product.deleted_at).not_to be_nil
-          expect(product.variants_including_master).to all(be_discarded)
+          expect(product.variants_including_master.reload).to all(be_discarded)
         end
       end
     end
diff --git a/core/spec/models/spree/variant_spec.rb b/core/spec/models/spree/variant_spec.rb
index 2e1e5a39a12..55ed9e45e89 100644
--- a/core/spec/models/spree/variant_spec.rb
+++ b/core/spec/models/spree/variant_spec.rb
@@ -616,7 +616,7 @@
       variant.discard
 
       expect(variant.images).to be_empty
-      expect(variant.stock_items).to be_empty
+      expect(variant.stock_items.reload).to be_empty
       expect(variant.prices).to be_empty
       expect(variant.currently_valid_prices).to be_empty
     end
diff --git a/frontend/spec/features/taxons_spec.rb b/frontend/spec/features/taxons_spec.rb
index 53721ddf727..cd01d20db38 100644
--- a/frontend/spec/features/taxons_spec.rb
+++ b/frontend/spec/features/taxons_spec.rb
@@ -4,9 +4,9 @@
 
 describe "viewing products", type: :feature, inaccessible: true do
   let!(:taxonomy) { create(:taxonomy, name: "Category") }
-  let!(:super_clothing) { taxonomy.root.children.create(name: "Super Clothing") }
-  let!(:t_shirts) { super_clothing.children.create(name: "T-Shirts") }
-  let!(:xxl) { t_shirts.children.create(name: "XXL") }
+  let!(:super_clothing) { taxonomy.root.children.create!(name: "Super Clothing", taxonomy: taxonomy) }
+  let!(:t_shirts) { super_clothing.children.create!(name: "T-Shirts", taxonomy: taxonomy) }
+  let!(:xxl) { t_shirts.children.create!(name: "XXL", taxonomy: taxonomy) }
   let!(:product) do
     product = create(:product, name: "Superman T-Shirt")
     product.taxons << t_shirts