Rework Quantifier initialization

We're going to use the param `stock_location_or_id` in the next commit

core/app/models/spree/stock/quantifier.rb

@@ -11,22 +11,16 @@ class Quantifier
       #        If unspecified it will check inventory in all available StockLocations
       def initialize(variant, stock_location_or_id = nil)
         @variant = variant
-        @stock_items = variant.stock_items.select do |stock_item|
-          if stock_location_or_id
-            stock_item.stock_location == stock_location_or_id ||
-              stock_item.stock_location_id == stock_location_or_id
-          else
-            stock_item.stock_location.active?
-          end
-        end
+        @stock_location_or_id = stock_location_or_id
+        @stock_items = get_stock_items
       end
 
       # Returns the total number of inventory units on hand for the variant.
       #
       # @return [Fixnum] number of inventory units on hand, or infinity if
       #   inventory is not tracked on the variant.
       def total_on_hand
-        if @variant.should_track_inventory?
+        if variant.should_track_inventory?
           stock_items.sum(&:count_on_hand)
         else
           Float::INFINITY
@@ -48,6 +42,21 @@ def backorderable?
       def can_supply?(required)
         total_on_hand >= required || backorderable?
       end
+
+      private
+
+      attr_reader :variant, :stock_location_or_id
+
+      def get_stock_items
+        variant.stock_items.select do |stock_item|
+          if stock_location_or_id
+            stock_item.stock_location == stock_location_or_id ||
+              stock_item.stock_location_id == stock_location_or_id
+          else
+            stock_item.stock_location.active?
+          end
+        end
+      end
     end
   end
 end

core/spec/models/spree/stock/quantifier_spec.rb

@@ -2,17 +2,16 @@
 
 require 'rails_helper'
 
-RSpec.shared_examples_for 'unlimited supply' do
-  it 'can_supply? any amount' do
-    expect(subject.can_supply?(1)).to eq true
-    expect(subject.can_supply?(101)).to eq true
-    expect(subject.can_supply?(100_001)).to eq true
-  end
-end
-
 module Spree
   module Stock
     RSpec.describe Quantifier, type: :model do
+      shared_examples_for 'unlimited supply' do
+        it 'can_supply? any amount' do
+          expect(subject.can_supply?(1)).to eq true
+          expect(subject.can_supply?(101)).to eq true
+          expect(subject.can_supply?(100_001)).to eq true
+        end
+      end
       let(:target_stock_location) { nil }
       let!(:stock_location) { create :stock_location_with_items }
       let!(:stock_item) { stock_location.stock_items.order(:id).first }