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Searching for child_xxx objects of recursively_embeds_many models

I am using Mogoid for my Ruby on Rails project. Since I have a Category class that needs to have unknown number of subcategories, I figured recursively_embeds_many relation is the best way to go. However I am confused about how to search for child objects. All children are the same object type as the parent and have their unique IDs but calling find method for that child ID returns no results.
Any thoughts on how to get this accomplished?

Thanks,
Alik