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

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 +.