Thread-friendly shared connection

after.rb

class ActiveRecord::Base
  mattr_accessor :shared_connection
  @@shared_connection = nil

  def self.connection
    @@shared_connection || ConnectionPool::Wrapper.new(:size => 1) { retrieve_connection }
  end
end
ActiveRecord::Base.shared_connection = ActiveRecord::Base.connection

before.rb

class ActiveRecord::Base
  mattr_accessor :shared_connection
  @@shared_connection = nil

  def self.connection
    @@shared_connection || retrieve_connection
  end
end
ActiveRecord::Base.shared_connection = ActiveRecord::Base.connection

gistfile1.txt

Capybara uses two threads to run client and server.  If you just cargo cult 
the DatabaseCleaner code that is floating around, you can run into 
https://github.com/brianmario/mysql2/issues/99 because the threads will
 occasionally use the same connection at the same time.

The fix is to use a single connection but protect access to it by having each
thread "check out" the connection when they are using it.  This is trivial to
do with the `connection_pool` gem.  Add it to your Gemfile and use the "after"
code above.