使用 capybara 和 poltergeist 运行 rspec 时随机出现以下错误：
NoMethodError: undefined method `result' for #<NoMethodError: undefined method `fields' for nil:NilClass>
我认为原因可能与数据库连接有关。那么，有人可以向我确认吗？我将尝试将截断与 database_cleaner 一起使用，但使用截断运行整个套件太慢了！
避免这种情况并保持规范快速运行的策略值得赞赏！
回溯：
~/.rvm/gems/ruby-2.0.0-p0/gems/activerecord-3.2.13/lib/active_record/connection_adapters/postgresql_adapter.rb:1147:in `translate_exception'
~/.rvm/gems/ruby-2.0.0-p0/gems/activerecord-3.2.13/lib/active_record/connection_adapters/abstract_adapter.rb:284:in `rescue in log'
~/.rvm/gems/ruby-2.0.0-p0/gems/activerecord-3.2.13/lib/active_record/connection_adapters/abstract_adapter.rb:280:in `log'
~/.rvm/gems/ruby-2.0.0-p0/gems/activerecord-3.2.13/lib/active_record/connection_adapters/postgresql_adapter.rb:659:in `exec_query'
~/.rvm/gems/ruby-2.0.0-p0/gems/activerecord-3.2.13/lib/active_record/connection_adapters/postgresql_adapter.rb:1256:in `select'
~/.rvm/gems/ruby-2.0.0-p0/gems/activerecord-3.2.13/lib/active_record/connection_adapters/abstract/database_statements.rb:18:in `select_all'
~/.rvm/gems/ruby-2.0.0-p0/gems/activerecord-3.2.13/lib/active_record/connection_adapters/abstract/query_cache.rb:61:in `block in select_all'
~/.rvm/gems/ruby-2.0.0-p0/gems/activerecord-3.2.13/lib/active_record/connection_adapters/abstract/query_cache.rb:75:in `cache_sql'
~/.rvm/gems/ruby-2.0.0-p0/gems/activerecord-3.2.13/lib/active_record/connection_adapters/abstract/query_cache.rb:61:in `select_all'
~/.rvm/gems/ruby-2.0.0-p0/gems/activerecord-3.2.13/lib/active_record/querying.rb:38:in `block in find_by_sql'
~/.rvm/gems/ruby-2.0.0-p0/gems/activerecord-3.2.13/lib/active_record/explain.rb:41:in `logging_query_plan'
~/.rvm/gems/ruby-2.0.0-p0/gems/activerecord-3.2.13/lib/active_record/querying.rb:37:in `find_by_sql'
~/.rvm/gems/ruby-2.0.0-p0/gems/activerecord-3.2.13/lib/active_record/relation.rb:171:in `exec_queries'
~/.rvm/gems/ruby-2.0.0-p0/gems/activerecord-3.2.13/lib/active_record/relation.rb:160:in `block in to_a'
~/.rvm/gems/ruby-2.0.0-p0/gems/activerecord-3.2.13/lib/active_record/explain.rb:41:in `logging_query_plan'
~/.rvm/gems/ruby-2.0.0-p0/gems/activerecord-3.2.13/lib/active_record/relation.rb:159:in `to_a'
~/.rvm/gems/ruby-2.0.0-p0/gems/activerecord-3.2.13/lib/active_record/relation/finder_methods.rb:380:in `find_first'
~/.rvm/gems/ruby-2.0.0-p0/gems/activerecord-3.2.13/lib/active_record/relation/finder_methods.rb:122:in `first'
~/.rvm/gems/ruby-2.0.0-p0/gems/orm_adapter-0.4.0/lib/orm_adapter/adapters/active_record.rb:17:in `get'
~/.rvm/gems/ruby-2.0.0-p0/gems/devise-2.2.3/lib/devise/models/authenticatable.rb:192:in `serialize_from_session'
~/.rvm/gems/ruby-2.0.0-p0/gems/devise-2.2.3/lib/devise/rails/warden_compat.rb:29:in `deserialize'
~/.rvm/gems/ruby-2.0.0-p0/gems/warden-1.2.1/lib/warden/session_serializer.rb:35:in `fetch'
~/.rvm/gems/ruby-2.0.0-p0/gems/warden-1.2.1/lib/warden/proxy.rb:212:in `user'
~/.rvm/gems/ruby-2.0.0-p0/gems/warden-1.2.1/lib/warden/proxy.rb:318:in `_perform_authentication'
~/.rvm/gems/ruby-2.0.0-p0/gems/warden-1.2.1/lib/warden/proxy.rb:104:in `authenticate'
~/.rvm/gems/ruby-2.0.0-p0/gems/devise-2.2.3/lib/devise/controllers/helpers.rb:56:in `current_admin_user'
./app/controllers/admin/base_controller.rb:5:in `user_for_paper_trail'
~/.rvm/gems/ruby-2.0.0-p0/gems/paper_trail-2.7.1/lib/paper_trail/controller.rb:59:in `set_paper_trail_whodunnit'
~/.rvm/gems/ruby-2.0.0-p0/gems/activesupport-3.2.13/lib/active_support/callbacks.rb:429:in `_run__385666484__process_action__495591669__callbacks'
~/.rvm/gems/ruby-2.0.0-p0/gems/activesupport-3.2.13/lib/active_support/callbacks.rb:405:in `__run_callback'
~/.rvm/gems/ruby-2.0.0-p0/gems/activesupport-3.2.13/lib/active_support/callbacks.rb:385:in `_run_process_action_callbacks'
~/.rvm/gems/ruby-2.0.0-p0/gems/activesupport-3.2.13/lib/active_support/callbacks.rb:81:in `run_callbacks'
~/.rvm/gems/ruby-2.0.0-p0/gems/actionpack-3.2.13/lib/abstract_controller/callbacks.rb:17:in `process_action'
~/.rvm/gems/ruby-2.0.0-p0/gems/actionpack-3.2.13/lib/action_controller/metal/rescue.rb:29:in `process_action'
~/.rvm/gems/ruby-2.0.0-p0/gems/actionpack-3.2.13/lib/action_controller/metal/instrumentation.rb:30:in `block in process_action'
~/.rvm/gems/ruby-2.0.0-p0/gems/activesupport-3.2.13/lib/active_support/notifications.rb:123:in `block in instrument'
~/.rvm/gems/ruby-2.0.0-p0/gems/activesupport-3.2.13/lib/active_support/notifications/instrumenter.rb:20:in `instrument'
~/.rvm/gems/ruby-2.0.0-p0/gems/activesupport-3.2.13/lib/active_support/notifications.rb:123:in `instrument'
~/.rvm/gems/ruby-2.0.0-p0/gems/actionpack-3.2.13/lib/action_controller/metal/instrumentation.rb:29:in `process_action'
~/.rvm/gems/ruby-2.0.0-p0/gems/actionpack-3.2.13/lib/action_controller/metal/params_wrapper.rb:207:in `process_action'
~/.rvm/gems/ruby-2.0.0-p0/gems/activerecord-3.2.13/lib/active_record/railties/controller_runtime.rb:18:in `process_action'
~/.rvm/gems/ruby-2.0.0-p0/gems/actionpack-3.2.13/lib/abstract_controller/base.rb:121:in `process'
~/.rvm/gems/ruby-2.0.0-p0/gems/actionpack-3.2.13/lib/abstract_controller/rendering.rb:45:in `process'
~/.rvm/gems/ruby-2.0.0-p0/gems/actionpack-3.2.13/lib/action_controller/metal.rb:203:in `dispatch'
~/.rvm/gems/ruby-2.0.0-p0/gems/actionpack-3.2.13/lib/action_controller/metal/rack_delegation.rb:14:in `dispatch'
~/.rvm/gems/ruby-2.0.0-p0/gems/actionpack-3.2.13/lib/action_controller/metal.rb:246:in `block in action'
~/.rvm/gems/ruby-2.0.0-p0/gems/actionpack-3.2.13/lib/action_dispatch/routing/route_set.rb:73:in `call'
~/.rvm/gems/ruby-2.0.0-p0/gems/actionpack-3.2.13/lib/action_dispatch/routing/route_set.rb:73:in `dispatch'
~/.rvm/gems/ruby-2.0.0-p0/gems/actionpack-3.2.13/lib/action_dispatch/routing/route_set.rb:36:in `call'
~/.rvm/gems/ruby-2.0.0-p0/gems/journey-1.0.4/lib/journey/router.rb:68:in `block in call'
~/.rvm/gems/ruby-2.0.0-p0/gems/journey-1.0.4/lib/journey/router.rb:56:in `each'
~/.rvm/gems/ruby-2.0.0-p0/gems/journey-1.0.4/lib/journey/router.rb:56:in `call'
~/.rvm/gems/ruby-2.0.0-p0/gems/actionpack-3.2.13/lib/action_dispatch/routing/route_set.rb:612:in `call'
~/.rvm/gems/ruby-2.0.0-p0/gems/omniauth-1.1.1/lib/omniauth/strategy.rb:177:in `call!'
~/.rvm/gems/ruby-2.0.0-p0/gems/omniauth-1.1.1/lib/omniauth/strategy.rb:157:in `call'
~/.rvm/gems/ruby-2.0.0-p0/gems/request_store-1.0.5/lib/request_store/middleware.rb:9:in `call'
~/.rvm/gems/ruby-2.0.0-p0/gems/sass-3.2.5/lib/sass/plugin/rack.rb:54:in `call'
~/.rvm/gems/ruby-2.0.0-p0/gems/warden-1.2.1/lib/warden/manager.rb:35:in `block in call'
~/.rvm/gems/ruby-2.0.0-p0/gems/warden-1.2.1/lib/warden/manager.rb:34:in `catch'
~/.rvm/gems/ruby-2.0.0-p0/gems/warden-1.2.1/lib/warden/manager.rb:34:in `call'
~/.rvm/gems/ruby-2.0.0-p0/gems/actionpack-3.2.13/lib/action_dispatch/middleware/best_standards_support.rb:17:in `call'
~/.rvm/gems/ruby-2.0.0-p0/gems/rack-1.4.5/lib/rack/etag.rb:23:in `call'
~/.rvm/gems/ruby-2.0.0-p0/gems/rack-1.4.5/lib/rack/conditionalget.rb:35:in `call'
~/.rvm/gems/ruby-2.0.0-p0/gems/actionpack-3.2.13/lib/action_dispatch/middleware/head.rb:14:in `call'
~/.rvm/gems/ruby-2.0.0-p0/gems/actionpack-3.2.13/lib/action_dispatch/middleware/params_parser.rb:21:in `call'
~/.rvm/gems/ruby-2.0.0-p0/gems/actionpack-3.2.13/lib/action_dispatch/middleware/flash.rb:242:in `call'
~/.rvm/gems/ruby-2.0.0-p0/gems/rack-1.4.5/lib/rack/session/abstract/id.rb:210:in `context'
~/.rvm/gems/ruby-2.0.0-p0/gems/rack-1.4.5/lib/rack/session/abstract/id.rb:205:in `call'
~/.rvm/gems/ruby-2.0.0-p0/gems/actionpack-3.2.13/lib/action_dispatch/middleware/cookies.rb:341:in `call'
~/.rvm/gems/ruby-2.0.0-p0/gems/activerecord-3.2.13/lib/active_record/query_cache.rb:64:in `call'
~/.rvm/gems/ruby-2.0.0-p0/gems/activerecord-3.2.13/lib/active_record/connection_adapters/abstract/connection_pool.rb:479:in `call'
~/.rvm/gems/ruby-2.0.0-p0/gems/actionpack-3.2.13/lib/action_dispatch/middleware/callbacks.rb:28:in `block in call'
~/.rvm/gems/ruby-2.0.0-p0/gems/activesupport-3.2.13/lib/active_support/callbacks.rb:405:in `_run__267372543__call__960205176__callbacks'
~/.rvm/gems/ruby-2.0.0-p0/gems/activesupport-3.2.13/lib/active_support/callbacks.rb:405:in `__run_callback'
~/.rvm/gems/ruby-2.0.0-p0/gems/activesupport-3.2.13/lib/active_support/callbacks.rb:385:in `_run_call_callbacks'
~/.rvm/gems/ruby-2.0.0-p0/gems/activesupport-3.2.13/lib/active_support/callbacks.rb:81:in `run_callbacks'
~/.rvm/gems/ruby-2.0.0-p0/gems/actionpack-3.2.13/lib/action_dispatch/middleware/callbacks.rb:27:in `call'
~/.rvm/gems/ruby-2.0.0-p0/gems/actionpack-3.2.13/lib/action_dispatch/middleware/remote_ip.rb:31:in `call'
~/.rvm/gems/ruby-2.0.0-p0/gems/actionpack-3.2.13/lib/action_dispatch/middleware/debug_exceptions.rb:16:in `call'
~/.rvm/gems/ruby-2.0.0-p0/gems/actionpack-3.2.13/lib/action_dispatch/middleware/show_exceptions.rb:56:in `call'
~/.rvm/gems/ruby-2.0.0-p0/gems/railties-3.2.13/lib/rails/rack/logger.rb:32:in `call_app'
~/.rvm/gems/ruby-2.0.0-p0/gems/railties-3.2.13/lib/rails/rack/logger.rb:16:in `block in call'
~/.rvm/gems/ruby-2.0.0-p0/gems/activesupport-3.2.13/lib/active_support/tagged_logging.rb:22:in `tagged'
~/.rvm/gems/ruby-2.0.0-p0/gems/railties-3.2.13/lib/rails/rack/logger.rb:16:in `call'
~/.rvm/gems/ruby-2.0.0-p0/gems/actionpack-3.2.13/lib/action_dispatch/middleware/request_id.rb:22:in `call'
~/.rvm/gems/ruby-2.0.0-p0/gems/rack-1.4.5/lib/rack/methodoverride.rb:21:in `call'
~/.rvm/gems/ruby-2.0.0-p0/gems/rack-1.4.5/lib/rack/runtime.rb:17:in `call'
~/.rvm/gems/ruby-2.0.0-p0/gems/activesupport-3.2.13/lib/active_support/cache/strategy/local_cache.rb:72:in `call'
~/.rvm/gems/ruby-2.0.0-p0/gems/rack-1.4.5/lib/rack/lock.rb:15:in `call'
~/.rvm/gems/ruby-2.0.0-p0/gems/actionpack-3.2.13/lib/action_dispatch/middleware/static.rb:63:in `call'
~/.rvm/gems/ruby-2.0.0-p0/gems/railties-3.2.13/lib/rails/engine.rb:479:in `call'
~/.rvm/gems/ruby-2.0.0-p0/gems/railties-3.2.13/lib/rails/application.rb:223:in `call'
~/.rvm/gems/ruby-2.0.0-p0/gems/rack-1.4.5/lib/rack/builder.rb:134:in `call'
~/.rvm/gems/ruby-2.0.0-p0/gems/rack-1.4.5/lib/rack/urlmap.rb:64:in `block in call'
~/.rvm/gems/ruby-2.0.0-p0/gems/rack-1.4.5/lib/rack/urlmap.rb:49:in `each'
~/.rvm/gems/ruby-2.0.0-p0/gems/rack-1.4.5/lib/rack/urlmap.rb:49:in `call'
~/.rvm/gems/ruby-2.0.0-p0/gems/capybara-2.1.0/lib/capybara/server.rb:19:in `call'
~/.rvm/gems/ruby-2.0.0-p0/gems/rack-1.4.5/lib/rack/handler/webrick.rb:59:in `service'
~/.rvm/rubies/ruby-2.0.0-p0/lib/ruby/2.0.0/webrick/httpserver.rb:138:in `service'
~/.rvm/rubies/ruby-2.0.0-p0/lib/ruby/2.0.0/webrick/httpserver.rb:94:in `run'
~/.rvm/rubies/ruby-2.0.0-p0/lib/ruby/2.0.0/webrick/server.rb:295:in `block in start_thread'# /home/rafaelsouza/.rvm/gems/ruby-2.0.0-p0/gems/activerecord-3.2.13/lib/active_record/connection_adapters/postgresql_adapter.rb:1147:in `translate_exception'