Associate customers with their user records

When we introduced the Customer model, we didn't associate any existing
customers with users that have the same email address.
Later we decided to create that association when users sign up. But we didn't
update all the existing customers. We do that now for data consistency and to
solve several bugs.

db/migrate/20181123012635_associate_customers_to_users.rb

@@ -0,0 +1,42 @@
+# When we introduced the Customer model, we didn't associate any existing
+# customers with users that have the same email address.
+# Later we decided to create that association when users sign up. But we didn't
+# update all the existing customers. We do that now for data consistency and to
+# solve several bugs.
+#
+# - https://github.com/openfoodfoundation/openfoodnetwork/pull/2084
+# - https://github.com/openfoodfoundation/openfoodnetwork/issues/2841
+class AssociateCustomersToUsers < ActiveRecord::Migration
+  class Customer < ActiveRecord::Base
+  end
+
+  def up
+    save_customers
+    execute "UPDATE customers
+              SET user_id = spree_users.id
+              FROM spree_users
+              WHERE customers.email = spree_users.email
+               AND customers.user_id IS NULL;"
+  end
+
+  def down
+    customers = backed_up_customers
+    Customer.where(id: customers).update_all(user_id: nil)
+  end
+
+  def save_customers
+    customers = Customer.
+      joins("INNER JOIN spree_users ON customers.email = spree_users.email").
+      where(user_id: nil).all
+
+    File.write(backup_file, Marshal.dump(customers))
+  end
+
+  def backed_up_customers
+    Marshal.load(File.read(backup_file))
+  end
+
+  def backup_file
+    File.join("log", "customers_without_user_association.log")
+  end
+end