shared_connection.rb

# In your test_helper.rb
class ActiveRecord::Base
  mattr_accessor :shared_connection
  @@shared_connection = nil

  def self.connection
    @@shared_connection || retrieve_connection
  end
end

# Forces all threads to share the same connection. This works on
# Capybara because it starts the web server in a thread.
ActiveRecord::Base.shared_connection = ActiveRecord::Base.connection

We should work on a reliable solution for everyone.

The best take I've seen so far is @iangreenleaf's article and gem:
http://technotes.iangreenleaf.com/posts/the-one-true-guide-to-database-transactions-with-capybara.html
https://github.com/iangreenleaf/transactional_capybara

It bundles the shared connection monkey patch and the wait_for_ajax helper.
Version 0.1.0 has just been released with support for Capybara >= 2.6.0.

It doesn't use connection_pool nor a Mutex.
I haven't had the need for it so far, but it's just a PR away.

Sorry to dig up an old thread (no pun intended), but I've been facing this issue on a Rails 5 app recently, and @mperham's solution worked great for me!

lol yeah this thread is filled with golden lessons in concurrency.

It’s 2019, do we still need this in Rails 5.2?

@gnclmorais, it seems that this is not useful anymore: https://github.com/teamcapybara/capybara#transactions-and-database-setup if you are running Rails 5.1 +.