UPDATE accounts SET account_type_id=subquery.id FROM (SELECT id FROM account_types WHERE mongo_id = accounts.mongo_account_type_id) AS subquery WHERE accounts.id = 1;

mongo_id                       | 521289ae1e0345000200000f
mongo_merchant_id              | 55e13f0afbab68982e000006
mongo_account_type_id          | 521289c41e03450002000011
mongo_admin_id                 | 518b762e89651a0389000013

mongo_id          | 521289c41e03450002000011
mongo_merchant_id | 55e13f0afbab68982e000006