"belongsTo" relation with additional condition for this model's table

Hello! The question is how to describe "belongsTo" relation with additional condition for this model's table. For example, where are 3 tables: "categories", "items" and "comments". Comments can be attached to items and categories, so we have fields "comments.category_id" and "comments.item_id". For unknown reason, if comment is attached to item, the both fields are set. So if I want to get category what comment belongs to, I have to make sure the "comments.item_id" field is null. How to properly describe category() relation in Comment model?