dbModel read resource does not implement Zend_Db_Adapter_Abstract

a:4:{i:0;s:65:"dbModel read resource does not implement Zend_Db_Adapter_Abstract";i:1;s:1555:"#0 /var/www/html/magento1.9.1/app/code/core/Mage/Core/Model/Resource/Db/Collection/Abstract.php(134): Varien_Data_Collection_Db->setConnection(false)
#1 /var/www/html/magento1.9.1/app/code/core/Mage/Core/Model/Config.php(1348): Mage_Core_Model_Resource_Db_Collection_Abstract->__construct(Object(Mage_Core_Model_Resource_Website))
#2 /var/www/html/magento1.9.1/app/code/core/Mage/Core/Model/Config.php(1380): Mage_Core_Model_Config->getModelInstance('core_resource/w...', Object(Mage_Core_Model_Resource_Website))
#3 /var/www/html/magento1.9.1/app/Mage.php(491): Mage_Core_Model_Config->getResourceModelInstance('core/website_co...', Object(Mage_Core_Model_Resource_Website))
#4 /var/www/html/magento1.9.1/app/code/core/Mage/Core/Model/Abstract.php(208): Mage::getResourceModel('core/website_co...', Object(Mage_Core_Model_Resource_Website))
#5 /var/www/html/magento1.9.1/app/code/core/Mage/Core/Model/Abstract.php(213): Mage_Core_Model_Abstract->getResourceCollection()
#6 /var/www/html/magento1.9.1/app/code/core/Mage/Core/Model/App.php(608): Mage_Core_Model_Abstract->getCollection()
#7 /var/www/html/magento1.9.1/app/code/core/Mage/Core/Model/App.php(466): Mage_Core_Model_App->_initStores()
#8 /var/www/html/magento1.9.1/app/code/core/Mage/Core/Model/App.php(349): Mage_Core_Model_App->_initCurrentStore('us_mylutionwal...', 'website')
#9 /var/www/html/magento1.9.1/app/Mage.php(684): Mage_Core_Model_App->run(Array)
#10 /var/www/html/magento1.9.1/index.php(87): Mage::run('us_mylutionwal...', 'website')
#11 {main}";s:3:"url";s:1:"/";s:11:"script_name";s:10:"/index.php";}

The solution is to clear everything in var/cache and make sure the permission is correct:

# rm -rf /www/magento1.9.1/var/cache/*
# find /www/magento1.9.1/ | xargs -I {} chown svn:apache {}
# find /www/magento1.9.1/ -type d | xargs -I {} chmod 770 {}
# find /www/magento1.9.1/ -type f | xargs -I {} chmod 660 {}