Query variant stock including overrides

This allows the results to be properly filtered and paginated whilst showing the correct stock, and removes a big N+1

lib/open_food_network/products_renderer.rb

@@ -34,17 +34,60 @@ module OpenFoodNetwork
 
       Spree::Product.where(id: distributed_products).
         order(taxon_order).
-        each { |product| scoper.scope(product) }.
-        select do |product|
-          product.has_stock_for_distribution?(@order_cycle, @distributor)
-        end
+        each { |product| scoper.scope(product) }
     end
 
     def distributed_products
       @order_cycle.
         variants_distributed_by(@distributor).
-        includes(:product).
-        select(:product_id)
+        merge(stocked_variants_with_overrides).
+        select("DISTINCT spree_variants.product_id")
+    end
+
+    def stocked_variants_with_overrides
+      Spree::Variant.
+        joins("LEFT OUTER JOIN variant_overrides ON variant_overrides.variant_id = spree_variants.id AND variant_overrides.hub_id = #{@distributor.id}").
+        joins(:stock_items).
+        where(query_stock_with_overrides)
+    end
+
+    def query_stock_with_overrides
+      "( #{variant_not_overriden} AND ( #{variant_in_stock} OR #{variant_on_demand} ) )
+        OR ( #{variant_overriden} AND ( #{override_on_demand} OR #{override_in_stock} ) )
+        OR ( #{variant_overriden} AND ( #{override_on_demand_null} AND #{variant_on_demand} ) )
+        OR ( #{variant_overriden} AND ( #{override_on_demand_null} AND #{variant_not_on_demand} AND #{variant_in_stock} ) )"
+    end
+
+    def variant_not_overriden
+      "variant_overrides.id IS NULL"
+    end
+
+    def variant_overriden
+      "variant_overrides.id IS NOT NULL"
+    end
+
+    def variant_in_stock
+      "spree_stock_items.count_on_hand > 0"
+    end
+
+    def variant_on_demand
+      "spree_stock_items.backorderable IS TRUE"
+    end
+
+    def variant_not_on_demand
+      "spree_stock_items.backorderable IS FALSE"
+    end
+
+    def override_on_demand
+      "variant_overrides.on_demand IS TRUE"
+    end
+
+    def override_in_stock
+      "variant_overrides.count_on_hand > 0"
+    end
+
+    def override_on_demand_null
+      "variant_overrides.on_demand IS NULL"
     end
 
     def scoper