Do not initialize promotions object on startup

api/lib/spree/api_configuration.rb

@@ -112,7 +112,17 @@ class ApiConfiguration < Preferences::Configuration
       :variant_id
     ]
 
-    preference :promotion_attributes, :array, default: Spree::Config.promotions.promotion_api_attributes
+    def promotion_attributes
+      Spree::Config.promotions.promotion_api_attributes
+    end
+    alias_method :preferred_promotion_attributes, :promotion_attributes
+
+    def promotion_attributes=(value)
+      Spree::Config.promotions.promotion_api_attributes = value
+    end
+    alias_method :preferred_promotion_attributes=, :promotion_attributes=
+    promotion_attributes_deprecation_message = "Spree::ApiConfiguration#promotion_attributes= is deprecated. Please use Spree::Config.promotions.promotion_api_attributes= instead."
+    deprecate "promotion_attributes=" => promotion_attributes_deprecation_message, deprecator: Spree.deprecator
 
     preference :store_attributes, :array, default: [
       :id, :name, :url, :meta_description, :meta_keywords, :seo_title,

api/spec/lib/spree/api_configuration_spec.rb

@@ -0,0 +1,41 @@
+# frozen_string_literal: true
+
+require "spec_helper"
+
+RSpec.describe Spree::ApiConfiguration do
+  subject(:config) { Spree::ApiConfiguration.new }
+
+  describe "#promotion_attributes" do
+    subject(:promotion_attributes) { config.promotion_attributes }
+
+    it { is_expected.to eq(Spree::Config.promotions.promotion_api_attributes) }
+
+    it "can be changed" do
+      config.promotion_attributes << :foo
+      expect(promotion_attributes).to include(:foo)
+    end
+
+    it "can delete attributes" do
+      expect(promotion_attributes).to include(:name)
+      config.promotion_attributes.delete(:name)
+      expect(promotion_attributes).not_to include(:name)
+    end
+  end
+
+  describe "#promotion_attributes=" do
+    subject(:promotion_attributes_setter) { config.promotion_attributes = [:name] }
+
+    around do |example|
+      original_attributes = Spree::Config.promotions.promotion_api_attributes
+      Spree.deprecator.silence do
+        example.run
+      end
+      Spree::Config.promotions.promotion_api_attributes = original_attributes
+    end
+
+    it "sets the promotion_attributes" do
+      promotion_attributes_setter
+      expect(config.promotion_attributes).to eq([:name])
+    end
+  end
+end