I’ve run this code
Zend_Loader::loadClass("Admin_Models_DbTable_News");
and my application has this folder structure

Why do I get this:
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Zend Framework ships with a concrete implementation of Zend_Loader_Autoloader_Resource that contains resource type mappings that cover the default recommended directory structure for Zend Framework MVC applications.
This loader, Zend_Application_Module_Autoloader, comes with the following mappings:
Given your example, your class should be called “Admin_Model_DbTable_News”. Note that the “s” is left out. This class should be put in application/modules/admin/models/DbTable/News.php