add unique constraint in model and database

app/models/carto/user_multifactor_auth.rb

@@ -13,6 +13,7 @@ module Carto
     belongs_to :user, inverse_of: :user_multifactor_auths, foreign_key: :user_id
 
     validates :type, inclusion: { in: VALID_TYPES }
+    validates_uniqueness_of :type, scope: :user_id
 
     before_create :create_shared_secret
 

db/migrate/20181108134709_add_unique_constraint_to_user_multifactor_auths.rb

@@ -0,0 +1,16 @@
+require 'carto/db/migration_helper'
+
+include Carto::Db::MigrationHelper
+
+migration(
+  Proc.new do
+    alter_table :user_multifactor_auths do
+      add_unique_constraint [:user_id, :type]
+    end
+  end,
+  Proc.new do
+    alter_table :user_multifactor_auths do
+      drop_constraint :user_multifactor_auths_user_id_type_key
+    end
+  end
+)