Simplify resetting variants

I was hoping to reduce the query count but it stayed the same. In fact,
I'm expecting the query count to be higher with this version. The
DfcCatalogImporter queries all variants and links at once while the OC
job is not pre-loading the variants at the moment. We can optimise the
job though.

If we kept the old version and there were multiple catalogs per variant
then we would call the importer with the reset multiple times. The job
is iterating through each link only once though. So depending on the
ratio of catalogs to variants, I'm not sure which version would be more
efficient.

No version is properly dealing with the edge-case of multiple catalogs
per variant anyway. Imagine there are two catalogs with different stock
levels. Which one do we choose? Or do we add up?
And if the variant disappears from one catalog we still want to sell it
through the other. But depending on the order of processing, we may
reset the variant if it's missing in the last catalog. But let's worry
about that when it actually happens. Maybe it will be better to restrict
variants to one catalog.

app/jobs/open_order_cycle_job.rb

@@ -57,14 +57,14 @@ class OpenOrderCycleJob < ApplicationJob
       catalog = DfcCatalog.load(dfc_user, catalog_url)
       catalog.apply_wholesale_values!
 
-      variants = Spree::Variant.where(id: catalog_links.pluck(:subject_id))
-      importer = DfcCatalogImporter.new(variants, catalog)
-      importer.reset_absent_variants
-
       catalog_links.each do |link|
         catalog_item = catalog.item(link.semantic_id)
 
-        SuppliedProductImporter.update_product(catalog_item, link.subject) if catalog_item
+        if catalog_item
+          SuppliedProductImporter.update_product(catalog_item, link.subject)
+        else
+          DfcCatalogImporter.reset_variant(link.subject)
+        end
       end
     end
   end

app/services/dfc_catalog_importer.rb

@@ -1,6 +1,14 @@
 # frozen_string_literal: true
 
 class DfcCatalogImporter
+  def self.reset_variant(variant)
+    if variant.on_demand
+      variant.on_demand = false
+    else
+      variant.on_hand = 0
+    end
+  end
+
   attr_reader :catalog, :existing_variants
 
   def initialize(existing_variants, catalog)
@@ -22,11 +30,7 @@ class DfcCatalogImporter
   # we don't want to lose the connection to previous orders.
   def reset_absent_variants
     absent_variants.map do |variant|
-      if variant.on_demand
-        variant.on_demand = false
-      else
-        variant.on_hand = 0
-      end
+      self.class.reset_variant(variant)
     end
   end