Tentative fix

Author: spaghetticode

core/db/migrate/20200117125833_add_merged_to_order_id.rb

@@ -1,7 +1,8 @@
 # frozen_string_literal: true
 
-class AddMergedToOrderId < ActiveRecord::Migration[5.1]
+class AddMergedToOrderId < ActiveRecord::Migration[5.2]
   def change
-    add_reference :spree_orders, :merged_to_order, foreign_key: { to_table: :spree_orders }
+    add_column :spree_orders, :merged_to_order_id, :integer, limit: 4
+    add_foreign_key :spree_orders, :spree_orders, column: :merged_to_order_id
   end
 end