Postgres: prepared statement already exists

Question

I use Devise for authentication in my Rails API app. Sometimes I see following error in the logs:

ActiveRecord::StatementInvalid: PGError: ERROR: prepared statement "a3926" already exists: UPDATE "users" SET "current_sign_in_at" = $1, "last_sign_in_at" = $2, "sign_in_count" = $3, "updated_at" = $4 WHERE "users"."id" = 12345

The error is coming out of Devise:

  def update_tracked_fields!(request)
    old_current, new_current = self.current_sign_in_at, Time.now.utc
    self.last_sign_in_at     = old_current || new_current
    self.current_sign_in_at  = new_current

    old_current, new_current = self.current_sign_in_ip, request.remote_ip
    self.last_sign_in_ip     = old_current || new_current
    self.current_sign_in_ip  = new_current

    self.sign_in_count ||= 0
    self.sign_in_count += 1

    # error happens below
    save(validate: false) or raise "Devise trackable could not save #{inspect}." \
      "Please make sure a model using trackable can be saved at sign in."
  end

As I understand this error usually happens when there is something wrong with how database connections are used. Is there something I should be looking for?

Looks like this might be a Rails bug: github.com/rails/rails/issues/1627#issuecomment-56219485 — Andrew Bezzub
@Marklar: yep, disabled prepared statements in my app. Rails code is not thread safe, so anything that can interrupt saving of the record can cause this error. In my case it was rack-timeout gem. — Andrew Bezzub

Mark Meeus Mark Meeus · Accepted Answer · 2014-10-21T19:30:37

If you are using a server which forks your process (like unicorn) you need to create a connection for every forked process.

In case of unicorn add this:

#config/unicorn.rb
before_fork do |server, worker|

  Signal.trap 'TERM' do
   puts 'Unicorn master intercepting TERM and sending myself QUIT instead'
   Process.kill 'QUIT', Process.pid
 end

  defined?(ActiveRecord::Base) and
    ActiveRecord::Base.connection.disconnect!
end

after_fork do |server, worker|

  Signal.trap 'TERM' do
    puts 'Unicorn worker intercepting TERM and doing nothing. Wait for master to sent QUIT'
  end

 defined?(ActiveRecord::Base) and
   ActiveRecord::Base.establish_connection(
     Rails.application.config.database_configuration[Rails.env]
   )

end

Postgres: prepared statement already exists

1 Answers