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I have extended Mage_Catalog_Model_Category_Attribute_Source_Mode (and it's Resource model) to add two new modes display_mode values for Categories. Now I want to validate the value of that attribute when saving a category. Specifically, I want to make sure that is_anchor is always set to "No" when either of my two new modes are selected.

The problem is that neither Mage_Eav_Model_Entity_Attribute_Backend_Abstract::beforeSave nor Mage_Eav_Model_Entity_Attribute_Backend_Abstract::validate dispatch any events that I can listen for.

Mage_Catalog_Model_Category also has no _beforeSave method.

How can I validate category attributes on save? Thanks!
