How to manualy commit or rollback in postgres ruby - ruby

Question

I had some ruby code in a ruby script that connected to a mysql DB. When I connected I then did a MySQL2.query("BEGIN") to start a transaction and if I wanted to rollback I did MySQL2.query("ROLLBACK") or if all was well and I wanted to commit I did MySQL2.query("COMMIT").
I have now moved to a postgres database and while the PG.exec("BEGIN"), PG.exec("ROLLBACK") and PG.exec("COMMIT") do not seem to error I do get a warning 'there is no transaction in progress' when I commit so it seems it is doing auto commit (i.e. committing each SQL INSERT/UPDATE as it is done). Basically I want to be able to manually rollback or commit.
I think maybe I need to turn autocommit off but cant work out how, I tried #dbase.exec('SET AUTOCOMMIT TO OFF') but got the error 'lib/database.rb:28:in `exec': ERROR: unrecognized configuration parameter "autocommit" (PG::UndefinedObject)'.
Ive done a fair amount of googling without any luck;(.
I am using postgres 9.5 and ruby 2.4.1.

Answer 1

PostgreSQL does not have a setting to disable autocommit:
https://stackoverflow.com/a/17936997/3323777
You just should use BEGIN/COMMIT/ROLLBACK.
BTW which adapter gem do you use? PG.exec syntax seems strange.
Consider the following snippet (pg 0.20 used):
conn = PGconn.open(:dbname => 'database', user: "user", password: "...")
conn.exec("DELETE FROM setting_entries")
conn.exec("INSERT INTO setting_entries(name) VALUES ('1')")
conn.exec("BEGIN")
conn.exec("DELETE FROM setting_entries")
conn.exec("INSERT INTO setting_entries(name) VALUES ('1')")
conn.exec("ROLLBACK")
And this is the postgre log:
(0-22/70) 2017-08-31 12:37:12 MSK [19945-1] user#database LOG: statement: DELETE FROM setting_entries
(0-22/71) 2017-08-31 12:37:12 MSK [19945-2] user#database LOG: statement: INSERT INTO setting_entries(name) VALUES ('1')
(0-22/72) 2017-08-31 12:37:12 MSK [19945-3] user#database LOG: statement: DELETE FROM setting_entries
(5948637-22/72) 2017-08-31 12:37:12 MSK [19945-4] user#database LOG: statement: INSERT INTO setting_entries(name) VALUES ('1')
As you can see, the transaction ids are the same (/72) for the last two lines.
To be sure you could write a unit test where you would make two updates within a transaction, rollback them and see if both would roll back.