Под «релевантным», вы имеете в виду последнее?
class Lending < ActiveRecord::Base
has_many :contract
attr_reader :current_contract
def initialize
@current_contract = Contract.New
end
...
end
class Contract < ActiveRecord::Base
has_one :lending
...
def before_delete
# update lending to the most relevant contract
# if this is the current_contract for parent lending
end
end