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

The solution using Mutex (thanks to @mikecmpbll) and wait_for_ajax (thanks to @divineforest) works well for me.

Here is another tip from me.

In some occation, the undefined method 'fields' error can occur even if you call wait_for_ajax after triggering an Ajax call.

In fact, we have to wait for all database accesses are completed, not just for ajax.

Suppose that you have following scenario among others:

scenario 'Update account' do
  visit edit_account_path
  fill_in 'account_name', with: 'foo'
  find('#update-account').click # Triggers an Ajax call
  wait_for_ajax
  user.reload
  expect(user.name).to eq('foo')
end

If any database access occurs after this scenario ends, we will get the undefined method 'fields' error on the next scenario.

To prevent this problem, we have to write another expectation that is fulfilled only after all database accesses are completed.

For example, if we know that a text appears on the browser screen at the end of scenario, we can write like this:

scenario 'Update account' do
  visit edit_account_path
  fill_in 'account_name', with: 'foo'
  find('#update-account').click # Triggers an Ajax call
  expect(page).to have_text('The account is updated.')
  user.reload
  expect(user.name).to eq('foo')
end

@kuroda - have you found a more systematic way to fix this issue? We're experiencing the same thing, and rather than run around fixing a bunch of tests, ideally there's a lower level way to address the problem.

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 +.