The code in the comment fails on concurrent inserts if done inside a transaction.
The fix is to force a savepoint to run so that if the database raises an unique violation exception. Otherwise, you'll get errors like:
```
(0.3ms) BEGIN
Cart Load (0.5ms) SELECT "carts".* FROM "carts" WHERE "carts"."uuid" = '12345' LIMIT 1
# Another process inserts a cart with uuid of '12345' right now
SQL (4371.7ms) INSERT INTO "carts" ("created_at", "updated_at", "uuid") VALUES ('2015-03-21 01:05:07.833231', '2015-03-21 01:05:07.833231', '12345') RETURNING "id" [["created_at", Sat, 21 Mar 2015 01:05:07 PDT -07:00], ["updated_at", Sat, 21 Mar 2015 01:05:07 PDT -07:00], ["uuid", "12345"]]
PG::UniqueViolation: ERROR: duplicate key value violates unique constraint "carts_uuid_idx1"
DETAIL: Key (uuid)=(12345) already exists.
: INSERT INTO "carts" ("created_at", "updated_at", "uuid") VALUES ('2015-03-21 01:05:07.833231', '2015-03-21 01:05:07.833231', '12345') RETURNING "id"
# Retrying the find
Cart Load (0.8ms) SELECT "carts".* FROM "carts" WHERE "carts"."uuid" = '12345' LIMIT 1
PG::InFailedSqlTransaction: ERROR: current transaction is aborted, commands ignored until end of transaction block
: SELECT "carts".* FROM "carts" WHERE "carts"."uuid" = '12345' LIMIT 1
(0.1ms) ROLLBACK
ActiveRecord::StatementInvalid: PG::InFailedSqlTransaction: ERROR: current transaction is aborted, commands ignored until end of transaction block
: SELECT "carts".* FROM "carts" WHERE "carts"."uuid" = '12345' LIMIT 1
```
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