Performance improvements on package generation

Right now, when Solidus tries to create the packages on Coordinator, it iterates over each one of the active stock locations and skip those without the requested variants in stock.

Let's suppose we have 100 different stock locations (which is my case) and the current user has 5 items on his cart. Once it tries to create the packages it triggers 500 different queries to the database.

Here I try to address this issue, iterating over just the stock locations with the requested variants in stock.

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

@@ -55,9 +55,8 @@ def build_location_configured_packages(packages = Array.new)
       #
       # Returns an array of Package instances
       def build_packages(packages = Array.new)
-        StockLocation.active.each do |stock_location|
+        stock_locations_with_requested_variants.each do |stock_location|
           units_for_location = unallocated_inventory_units.select { |unit| stock_location.stock_item(unit.variant) }
-          next unless units_for_location.any?
           packer = build_packer(stock_location, units_for_location)
           packages += packer.packages
         end
@@ -70,6 +69,15 @@ def unallocated_inventory_units
         inventory_units - @preallocated_inventory_units
       end
 
+      def stock_locations_with_requested_variants
+        Spree::StockLocation.active.joins(:stock_items).
+          where(spree_stock_items: { variant_id: unallocated_variant_ids }).uniq
+      end
+
+      def unallocated_variant_ids
+        unallocated_inventory_units.map(&:variant_id).uniq
+      end
+
       def prioritize_packages(packages)
         prioritizer = Prioritizer.new(inventory_units, packages)
         prioritizer.prioritized_packages