How to make ActiveRecord ThreadSafe

Question

How can I make the following controller threadsafe in rails 4 with postgresql:

def controller_action
  if Model.exists(column_name:\"some_value\")
  else
            


        

Answer 1

Contrary to the comments, concurrent inserts on the same table are entirely permissible in PostgreSQL, so there's a race condition here.

To make this safe you must have a unique constraint (or primary key) on column_name. Duplicate inserts will then throw an exception which you can catch and retry with an update.

If you don't have a unique constraint, then you must LOCK TABLE ... IN EXCLUSIVE MODE to prevent concurrent upserts. Or use one of the concurrency-safe methods described in:

How to UPSERT (MERGE, INSERT ... ON DUPLICATE UPDATE) in PostgreSQL?