set_belongs_to_association_for

# File activerecord/lib/active_record/associations/association_proxy.rb, line 179
        def set_belongs_to_association_for(record)
          if @reflection.options[:as]
            record["#{@reflection.options[:as]}_id"]   = @owner.id unless @owner.new_record?
            record["#{@reflection.options[:as]}_type"] = @owner.class.base_class.name.to_s
          else
            unless @owner.new_record?
              primary_key = @reflection.options[:primary_key] || :id
              record[@reflection.primary_key_name] = @owner.send(primary_key)
            end
          end
        end