Set required line item attributes earlier

This makes working with unpersisted line items easier because required
attributes are set immediately. This change is in service of creating an
in memory order updater.

Author: senemsoy

core/app/models/spree/line_item.rb

@@ -20,7 +20,7 @@ class LineItem < Spree::Base
     has_many :inventory_units, inverse_of: :line_item
 
     before_validation :normalize_quantity
-    before_validation :set_required_attributes
+    after_initialize :set_required_attributes
 
     validates :variant, presence: true
     validates :quantity, numericality: {
@@ -157,6 +157,7 @@ def normalize_quantity
     # Sets tax category, price-related attributes from
     # its variant if they are nil and a variant is present.
     def set_required_attributes
+      return if persisted?
       return unless variant
       self.tax_category ||= variant.tax_category
       set_pricing_attributes

core/spec/models/spree/line_item_spec.rb

@@ -39,31 +39,25 @@
     end
   end
 
-  describe 'line item creation' do
+  describe '.new' do
     let(:variant) { create :variant }
 
     subject(:line_item) { Spree::LineItem.new(variant:, order:) }
 
-    # Tests for https://github.com/spree/spree/issues/3391
-    context 'before validation' do
-      before { line_item.valid? }
-
-      it 'copies the variants price' do
-        expect(line_item.price).to eq(variant.price)
-      end
+    it 'copies the variants price' do
+      expect(line_item.price).to eq(variant.price)
+    end
 
-      it 'copies the variants cost_price' do
-        expect(line_item.cost_price).to eq(variant.cost_price)
-      end
+    it 'copies the variants cost_price' do
+      expect(line_item.cost_price).to eq(variant.cost_price)
+    end
 
-      it "copies the order's currency" do
-        expect(line_item.currency).to eq(order.currency)
-      end
+    it "copies the order's currency" do
+      expect(line_item.currency).to eq(order.currency)
+    end
 
-      # Test for https://github.com/spree/spree/issues/3481
-      it 'copies the variants tax category' do
-        expect(line_item.tax_category).to eq(line_item.variant.tax_category)
-      end
+    it 'copies the variants tax category' do
+      expect(line_item.tax_category).to eq(line_item.variant.tax_category)
     end
   end