Rails - how do I validate existence of a row referenced by foreign key

Given that the "Rails Way" seems to be not to use foreign key constraints, I'm looking for an alternative that will allow me to validate that the row a foreign key references does in fact exist in TableA before I save an object in TableB with table_a_id. The only resource I've found to do this so far (can't find a link to the blog post it was mentioned in, was dated 2007) doesn't appear to be comp