When testing with capybara

test_helper_part.rb

# Make all database transactions use the same thread
# This is needed so tests wich uses webkit (javascript enabled feature testing) also use the same thread for database transactions
# Furthermore this is needed so db changes made in these feature tests are able to be rolled back
ActiveRecord::ConnectionAdapters::ConnectionPool.class_eval do
  def current_connection_id
    Thread.main.object_id
  end
end