Improve naming in availability_validator_decorator and cover it with tests

app/models/spree/stock/availability_validator_decorator.rb

@@ -3,10 +3,10 @@ Spree::Stock::AvailabilityValidator.class_eval do
     # OFN specific check for in-memory :skip_stock_check attribute
     return if line_item.skip_stock_check
 
-    quantity = adapt_line_item_quantity_to_inventory_units(line_item)
-    return if quantity == 0
+    quantity_to_validate = line_item.quantity - quantity_in_shipment(line_item)
+    return if quantity_to_validate < 1
 
-    validate_quantity(line_item, quantity)
+    validate_quantity(line_item, quantity_to_validate)
   end
 
   private
@@ -14,12 +14,12 @@ Spree::Stock::AvailabilityValidator.class_eval do
   # This is an adapted version of a fix to the inventory_units not being considered here.
   # See #3090 for details.
   # This can be removed after upgrading to Spree 2.4.
-  def adapt_line_item_quantity_to_inventory_units(line_item)
+  def quantity_in_shipment(line_item)
     shipment = line_item_shipment(line_item)
-    return line_item.quantity unless shipment
+    return 0 unless shipment
 
     units = shipment.inventory_units_for(line_item.variant)
-    line_item.quantity - units.count
+    units.count
   end
 
   def line_item_shipment(line_item)

spec/models/spree/stock/availability_validator_spec.rb

@@ -0,0 +1,75 @@
+require 'spec_helper'
+
+module Spree
+  module Stock
+    describe AvailabilityValidator do
+      let(:validator) { AvailabilityValidator.new({}) }
+
+      context "line item without existing inventory units" do
+        let(:order) { create(:order_with_line_items) }
+        let(:line_item) { order.line_items.first }
+
+        before do
+          expect(order.shipment.inventory_units).to be_empty
+          expect(line_item.target_shipment).to be_nil
+        end
+
+        context "available quantity when variant.on_hand > line_item.quantity" do
+          it "suceeds" do
+            line_item.quantity = line_item.variant.on_hand - 1
+            validator.validate(line_item)
+            expect(line_item.errors[:quantity].size).to eq(0)
+          end
+        end
+
+        context "unavailable quantity when variant.on_hand < line_item.quantity" do
+          it "fails" do
+            line_item.quantity = line_item.variant.on_hand + 1
+            validator.validate(line_item)
+            expect_product_out_of_stock_error
+          end
+
+          it "succeeds with line_item skip_stock_check" do
+            line_item.skip_stock_check = true
+            line_item.quantity = line_item.variant.on_hand + 1
+            validator.validate(line_item)
+            expect(line_item.errors[:quantity].size).to eq(0)
+          end
+        end
+      end
+
+      describe "line item with existing inventory units" do
+        let(:order) { create(:completed_order_with_totals) }
+        let(:line_item) { order.line_items.first }
+
+        before do
+          expect(line_item.quantity).to eq(1)
+          expect(line_item.variant.on_hand).to eq(4)
+
+          expect(order.shipment.inventory_units).to_not be_empty
+          expect(order.shipment.inventory_units_for(line_item.variant).size).to eq(1)
+        end
+
+        context "when adding all variant.on_hand quantity to existing line_item quantity" do
+          it "succeeds because it excludes existing inventory units from the validation" do
+            line_item.quantity += line_item.variant.on_hand
+            validator.validate(line_item)
+            expect(line_item.errors[:quantity].size).to eq(0)
+          end
+
+          it "fails if one more item is added" do
+            line_item.quantity += line_item.variant.on_hand + 1
+            validator.validate(line_item)
+            expect_product_out_of_stock_error
+          end
+        end
+      end
+
+      def expect_product_out_of_stock_error
+        quantity_error = line_item.errors[:quantity].first
+        expect(quantity_error).to include(line_item.variant.product.name)
+        expect(quantity_error).to include("out of stock")
+      end
+    end
+  end
+end