I have a model for which I have provided a customer finder method, ''find_one_by_date(params)'', where params is a hash. This method always returns one or no record from the DB. I would like to define an association, has_one, from another model to this one, passing the params hash and using the custom finder that I have written. It seems to me as if this is not possible from my reading of the api. Before I try and roll my own solution to this I would like to know if it is, in fact, possible to do what I want through the has_one association in AR; and, if so, how is it done?