Fix add_uniqueness_of_order_id_to_spree_shipments by first removing shipments from orders with more than one shipment

db/migrate/20180910155506_add_uniqueness_of_order_id_to_spree_shipments.rb

@@ -1,6 +1,23 @@
+# This migration is an OFN specific migration that enforces an order to have a single shipment at all times
 class AddUniquenessOfOrderIdToSpreeShipments < ActiveRecord::Migration
   def change
+    destroy_all_but_latest_shipments
+
     remove_index :spree_shipments, :order_id
     add_index :spree_shipments, :order_id, unique: true
   end
+
+  private
+
+  # Destroy all but the latest shipment in each order
+  def destroy_all_but_latest_shipments
+    latest_shipments = Spree::Shipment.
+      select("order_id, MAX(updated_at) updated_at").
+      group(:order_id).
+      having("count(*) > 1")
+
+    Spree::Shipment.
+      joins("INNER JOIN (#{latest_shipments.to_sql}) latest_shipments ON spree_shipments.order_id=latest_shipments.order_id AND spree_shipments.updated_at != latest_shipments.updated_at").
+      destroy_all
+  end
 end