CODEWITHSUNDEEP
ruby-on-railsrubymultithreading
Taylor Allred
Taylor Allred

Reputation: 4380

Ruby Multi-Threading Sql (in test environment)

I have a problem in my tests running some SQL queries in different threads. I expect that running simple select and counts in SQL would return the same results, but if I run these methods in threads (process_new_followers and process_lost_followers) they return different results.

Here is the (basic) code:

def compute_followers
  followers = Follower.where({ user_id: user_id, is_following: true }).count # => returns 10 always

  calculate_differences
  thread1 = Thread.new { process_new_followers }
  thread2 = Thread.new { process_lost_followers }
  thread1.join
  thread2.join

end

def process_new_followers
  puts Follower.where({ user_id: user_id, is_following: true }).count # => returns 0 if threaded and 10 if not
  ... I commented out this code so it only prints the count and nothing changes, so no db changes are happening in this method
end

def process_lost_followers
  puts Follower.where({ user_id: user_id, is_following: true }).count # => returns 0 if threaded and 10 if not
  ... I commented out this code so it only prints the count and nothing changes, so no db changes are happening in this method
end

Why would the threading effect this SQL query?

Upvotes: 1

Views: 282

Answers (1)

Ben Eggett
Ben Eggett

Reputation: 486

Since this is in a Rails app using ActiveRecord, Rails will rollback all transactions in it's test suite. Since you're stepping outside of it in a thread, the transaction has most likely already rolled back, which would explain why you're seeing 0 count in threaded and 10 in non-threaded call.

Upvotes: 2

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