Further optimise querying affected exchanges

The implementation queried the database for each incoming variant that
was changed. This rewrite combines ActiveRecord relations so that it
creates only one query. This saves another 5-10% of execution time when
updating enterprise fees on production instances.

app/models/exchange.rb

@@ -38,7 +38,7 @@ class Exchange < ActiveRecord::Base
   }
   scope :with_any_variant, lambda { |variant_ids|
     joins(:exchange_variants).
-      where('exchange_variants.variant_id IN (?)', variant_ids).
+      where(exchange_variants: { variant_id: variant_ids }).
       select('DISTINCT exchanges.*')
   }
   scope :with_product, lambda { |product|

lib/open_food_network/products_cache.rb

@@ -165,13 +165,13 @@ module OpenFoodNetwork
     private_class_method :incoming_exchanges
 
     def self.outgoing_exchanges_affected_by(exchanges)
-      incoming_exchanges(exchanges).map do |incoming_exchange|
-        outgoing_exchanges_with_variants(
-          incoming_exchange.order_cycle,
-          incoming_exchange.variant_ids
-        )
-      end.flatten.uniq { |ex| [ex.receiver_id, ex.order_cycle_id]}
-      # Comparing only on these two ids is much faster.
+      incoming = incoming_exchanges(exchanges)
+      order_cycle_ids = incoming.select(:order_cycle_id)
+      variant_ids = ExchangeVariant.where(exchange_id: incoming).select(:variant_id)
+      Exchange.outgoing.
+        in_order_cycle(order_cycle_ids).
+        with_any_variant(variant_ids).
+        except(:select).select("DISTINCT receiver_id, order_cycle_id")
     end
     private_class_method :outgoing_exchanges_affected_by