This class (for yii-framework) can be used for search by virtual attributes (for example, in ajax filter of CGridView widget).
Value of the virtual attribute is dynamically calculated by method (so-called "virtual getter"), when you read this attribute. This class creates "search cache" in DB (adds column in table) and updates him after each updating of record. The said "search cache" can be used for forming of search criteria (like ordinary AR-attributes).
Detailed description with examples you can find in phpDoc (ActiveRecordVirtualAttribute.php).