Deal with PostgreSQL concurrency using Rails find_or_create - ruby-on-rails

Question

For some reason this code can create duplicated games if different users run it at the same moment:
game = Game.find_or_create_by(
status: Game::STATUS[:waiting],
category_id: params[:category_id],
private: 0
) do |g|
is_new = true
g.user = current_user
end
I can't figure out clearly what is the matter, but probably its about different Unicorn processes which use different database connections so transactions can run in parallel.
If so, I need the right way to avoid it, maybe I should use Rails transactions or Postgres locks, but I really need an example of using.
Thank you.

Answer 1

It can happen in high concurrency levels.
According to rails docs, these queries will run:
SELECT * FROM games WHERE status = 'waiting' AND ... LIMIT 1;
INSERT INTO games (status, ...) VALUES ('waiting', ...);
The second only runs, when the first haven't returned a row.
It is possible, if two (or more) connections starts the first query within a few microseconds, that multiple processes will create multiple entries. To prevent that, you can use an ACCESS EXCLUSIVE lock on that table, or use a custom advisory lock.
You can use some unique index too, to prevent multiple entries to be inserted into your database, but if it's used on its own, that will cause SQL exceptions in this situation.
EDIT:
ACCESS EXCLUSIVE lock can be acquired through the LOCK command.
Advisory lock can be acquired through using the pg_advisory_lock(id) function.
Both requires you to run arbitrary SQL commands.
Another way would be to use custom queries with:
insert only if it's not exists (& return with all fields)
select only if not inserted
Something, like:
INSERT INTO games (status, category_id, private)
SELECT 'waiting', 2, 0
WHERE NOT EXISTS (
SELECT 1
FROM games
WHERE status = 'waiting'
AND category_id = 2
AND private = 0
)
RETURNING *;
-- only select, when this not inserted anything
SELECT *
FROM games
WHERE status = 'waiting'
AND category_id = 2
AND private = 0