Serialize DFC catalog item with DFC Connector

This is work in progress. The DFC Connector uses a more recent DFC
version and other endpoints still use the old serializers. We need to
update those endpoints as well and update the version number in the API
URL.

engines/dfc_provider/app/controllers/dfc_provider/catalog_items_controller.rb

@@ -15,7 +15,8 @@ module DfcProvider
     end
 
     def show
-      render json: variant, serializer: DfcProvider::CatalogItemSerializer
+      dfc_object = DfcBuilder.catalog_item(variant)
+      render json: DfcLoader.connector.export(dfc_object)
     end
 
     private

engines/dfc_provider/app/services/dfc_builder.rb

@@ -0,0 +1,47 @@
+# frozen_string_literal: true
+
+class DfcBuilder
+  def self.catalog_item(variant)
+    id = urls.enterprise_catalog_item_url(
+      enterprise_id: variant.product.supplier_id,
+      id: variant.id,
+    )
+    product = supplied_product(variant)
+
+    DataFoodConsortium::Connector::CatalogItem.new(
+      id, product: product,
+          sku: variant.sku,
+          offers: [offer(variant)],
+    )
+  end
+
+  def self.supplied_product(variant)
+    id = urls.enterprise_supplied_product_url(
+      enterprise_id: variant.product.supplier_id,
+      id: variant.id,
+    )
+
+    DataFoodConsortium::Connector::SuppliedProduct.new(
+      id, name: variant.name, description: variant.description
+    )
+  end
+
+  def self.offer(variant)
+    id = "/offers/#{variant.id}"
+    offered_to = []
+
+    # The DFC sees "empty" stock as unlimited.
+    # http://static.datafoodconsortium.org/conception/DFC%20-%20Business%20rules.pdf
+    stock = variant.on_demand ? nil : variant.total_on_hand
+
+    DataFoodConsortium::Connector::Offer.new(
+      id, offeredTo: offered_to,
+          price: variant.price.to_f,
+          stockLimitation: stock,
+    )
+  end
+
+  def self.urls
+    DfcProvider::Engine.routes.url_helpers
+  end
+end

engines/dfc_provider/spec/services/offer_builder_spec.rb

@@ -0,0 +1,30 @@
+# frozen_string_literal: true
+
+require DfcProvider::Engine.root.join("spec/spec_helper")
+
+describe DfcBuilder do
+  let(:variant) { build(:variant) }
+
+  describe ".offer" do
+    it "assigns a stock level" do
+      # Assigning stock only works with persisted records:
+      variant.save!
+      variant.on_hand = 5
+
+      offer = DfcBuilder.offer(variant)
+
+      expect(offer.stockLimitation).to eq 5
+    end
+
+    it "has no stock limitation when on demand" do
+      # Assigning stock only works with persisted records:
+      variant.save!
+      variant.on_hand = 5
+      variant.on_demand = true
+
+      offer = DfcBuilder.offer(variant)
+
+      expect(offer.stockLimitation).to eq nil
+    end
+  end
+end