civicrm/civicrm-core — pull request #2067 by deepak-srivastava (2013-11-21 10:16:49+00:00)
Title: Allow custom table join clause to override

Implementation example : when we want to Include Campaign custom fields in contribution report,

The contribution report could just say:
+ protected $_customGroupExtends = array('Campaign');
+ protected $_customGroupExtendsJoin = array('Campaign' => 'contribution_civireport.campaign_id');