custom finder in associated model.

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?