Atomic INSERT or UPDATE with activerecord

Rails
1

what i need:
if :record is in the database -> upgrade his counter
else create :record (with his counter on 1)

this should be done with an atomic operation, to avoid race
conditions.
What is the best way to do this with activerecord?
I read about transactions, but as i understand a transaction acts on a
single connection to the database.

2

On Nov 8, 10:41 am, “[email protected]”
[email protected] wrote:

what i need:
if :record is in the database → upgrade his counter
else create :record (with his counter on 1)

this should be done with an atomic operation, to avoid race
conditions.
What is the best way to do this with activerecord?
I read about transactions, but as i understand a transaction acts on a
single connection to the database.

incrementing the counter is simple enough - AR’s increment_counter
method basically does update foos set counter = counter + 1 where …

To avoid duplicate records being created I’d create a unique index on
whatever it is that identifies a record

Fred

3

[email protected] wrote:

what i need:
if :record is in the database → upgrade his counter
else create :record (with his counter on 1)

this should be done with an atomic operation, to avoid race
conditions.
What is the best way to do this with activerecord?
I read about transactions, but as i understand a transaction acts on a
single connection to the database.

This would be a single DB connection, so it seems like a good candidate
for a transaction. What’s the problem with doing that?

Best,

Marnen Laibow-Koser
http://www.marnen.org
[email protected]

4

On Nov 8, 6:50 pm, Marnen Laibow-Koser <rails-mailing-l…@andreas-
s.net> wrote:

This would be a single DB connection, so it seems like a good candidate
for a transaction. What’s the problem with doing that?
i think that multiple requests use multiple db connections, didn’t
they? This is obviously true having two database server in a multi-
master configuration. I think that Frederick C.'s solution should
solve the problem anyway.
thank you.

5

eugenio wrote:

On Nov 8, 6:50�pm, Marnen Laibow-Koser <rails-mailing-l…@andreas-
s.net> wrote:

This would be a single DB connection, so it seems like a good candidate
for a transaction. �What’s the problem with doing that?
i think that multiple requests use multiple db connections, didn’t
they?

Of course. But you shouldn’t need a transaction over multiple requests;
just encapsulating the INSERT and counter increment in a transaction,
within a single request cycle, should do the trick.

This is obviously true having two database server in a multi-
master configuration.

Well, that’s just asking for trouble. :slight_smile:

I think that Frederick C.'s solution should
solve the problem anyway.
thank you.

Best,

Marnen Laibow-Koser
http://www.marnen.org
[email protected]