class ActiveRecord::ConnectionAdapters::ConnectionPool

def with_connection

connection when finished.
exists checkout a connection, yield it to the block, and checkin the
already exists yield it to the block. If no such connection
If a connection obtained through #connection or #with_connection methods 
def with_connection
  unless conn = @thread_cached_conns[connection_cache_key(Thread.current)]
    conn = connection
    fresh_connection = true
  end
  yield conn
ensure
  release_connection if fresh_connection
end