Efficiently finding elements with 0 has_many elements?

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Say I have a class Item and a class Data; each Item has_many Data, and
each Data has_a Item.

Is there an efficient way to find the rows of Item which have no Data?
I can do a retrieve_all on Item and iterate through them looking for
which ones have an empty $item->data, but it seems like I ought to be
able to do this operation in the database.

--dave