在启动时查询注册表时的ComponentLookupError

我正在构建一个应用程序,使用collective.lead(trunk)查询外部关系数据库中的某些数据.用户可以在自定义Plone控制面板工具中修改数据库连接设置(我按照Aspeli的Professional Plone Development书中的示例).以这种方式查询数据库设置.

我的产品base.zcml为数据库设置了一个实用程序：

<include package="plone.app.registry" />
<include package="collective.lead" />

<i18n:registerTranslations directory="locales" />

<utility
  provides="collective.lead.interfaces.IDatabase"
  factory=".dbsettings.CalculatorDatabase"
  name="test.calc.db"
  />

dbsettings.py有：

from zope.component import getUtility
from plone.registry.interfaces import IRegistry

class CalculatorDatabase(Database):

    @property
    def _url(self):
        registry = getUtility(IRegistry)
        settings = registry.forInterface(IDatabaseSettings)
        return URL(
            drivername=settings.drivername,username=settings.username,password=settings.password,host=settings.hostname,port=settings.port,database=settings.database,)

这会在运行时引发ComponentLookupError异常：

File "/home/zope/envs/test-web/src/test.calc/test/calc/dbsettings.py",line 38,in _url
    registry = getUtility(IRegistry)
File "/home/zope/envs/test-web/eggs/zope.component-3.7.1-py2.6.egg/zope/component/_api.py",line 171,in getUtility
    raise ComponentLookupError(interface,name)
zope.configuration.config.ConfigurationExecutionError: <class 'zope.component.interfaces.ComponentLookupError'>: (<InterfaceClass plone.registry.interfaces.IRegistry>,'')
in:
File "/home/zope/envs/test-web/src/test.calc/test/calc/configure.zcml",line 26.2-30.6
<utility
  provides="collective.lead.interfaces.IDatabase"
  factory=".dbsettings.CalculatorDatabase"
  name="test.calc.db"
  />

为什么在运行时找不到注册表？我究竟做错了什么？

谢谢.

<subscriber
    for="Products.CMFPlone.interfaces.IPloneSiteRoot
         zope.app.publication.interfaces.IBeforeTraverseEvent"
    handler=".component.setupStuff"
    />