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
    @model=Model.new(column_name:"some_value")
    @model.save
  end
end

I am running puma, so my concern is that if two threads run this controller at the same time, and a row doesn't exist with the specified value of column_name, two records will be created whereas I only want 1.

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?