Test stock logic on variant level

VariantOverrides are bolted onto variants to change their logic.

spec/models/spree/variant_stock_spec.rb

@@ -0,0 +1,89 @@
+# frozen_string_literal: false
+
+require 'spec_helper'
+
+RSpec.describe Spree::Variant do
+  # This method is defined in app/models/concerns/variant_stock.rb.
+  # There is a separate spec for that concern but here I want to test
+  # the interplay of Spree::Variant and VariantOverride.
+  #
+  # A variant can be scoped to a hub which means that all stock methods
+  # like this one get overridden. Future calls to `variant.move` are then
+  # handled by the ScopeVariantToHub module which may call the
+  # VariantOverride.
+  describe "#move" do
+    subject(:variant) { create(:variant, on_hand: 5) }
+
+    it "changes stock" do
+      expect { variant.move(-2) }.to change { variant.on_hand }.from(5).to(3)
+    end
+
+    it "ignores stock when on demand" do
+      variant.on_demand = true
+
+      expect { variant.move(-2) }.not_to change { variant.on_hand }
+    end
+
+    it "rejects negative stock" do
+      expect { variant.move(-7) }.to raise_error(
+        ActiveRecord::RecordInvalid,
+        "Validation failed: Count on hand must be greater than or equal to 0"
+      )
+    end
+
+    describe "with VariantOverride" do
+      subject(:hub_variant) {
+        Spree::Variant.find(variant.id).tap { |v| scoper.scope(v) }
+      }
+      let(:override) {
+        VariantOverride.create!(
+          variant:,
+          hub: create(:distributor_enterprise),
+          count_on_hand: 7,
+          on_demand: false,
+        )
+      }
+      let(:scoper) { OpenFoodNetwork::ScopeVariantToHub.new(override.hub) }
+
+      it "changes stock only on the variant override" do
+        expect {
+          hub_variant.move(-3)
+          override.reload
+        }
+          .to change { override.count_on_hand }.from(7).to(4)
+          .and change { hub_variant.on_hand }.from(7).to(4)
+          .and change { variant.on_hand }.by(0)
+      end
+
+      it "ignores stock when on demand" do
+        override.update!(on_demand: true, count_on_hand: nil)
+
+        expect {
+          hub_variant.move(-3)
+          override.reload
+        }
+          .not_to change {
+            [
+              override.count_on_hand,
+              hub_variant.on_hand,
+              variant.on_hand,
+            ]
+          }
+      end
+
+      it "doesn't prevent negative stock" do
+        # VariantOverride relies on other stock checks during checkout. :-(
+        expect {
+          hub_variant.move(-8)
+          override.reload
+        }
+          .to change { override.count_on_hand }.from(7).to(-1)
+          .and change { hub_variant.on_hand }.from(7).to(-1)
+          .and change { variant.on_hand }.by(0)
+
+        # The update didn't run validations and now it's invalid:
+        expect(override).not_to be_valid
+      end
+    end
+  end
+end