replace

# File activerecord/lib/active_record/associations/belongs_to_association.rb, line 10
      def replace(record)
        if record
          raise_on_type_mismatch!(record)
          update_counters(record)
          replace_keys(record)
          set_inverse_instance(record)
          @updated = true
        else
          decrement_counters
          remove_keys
        end

        self.target = record
      end