instance method writer_method

# File activerecord/lib/active_record/aggregations.rb, line 261
          def writer_method(name, class_name, mapping, allow_nil, converter)
            define_method("#{name}=") do |part|
              klass = class_name.constantize

              unless part.is_a?(klass) || converter.nil? || part.nil?
                part = converter.respond_to?(:call) ? converter.call(part) : klass.send(converter, part)
              end

              hash_from_multiparameter_assignment = part.is_a?(Hash) &&
                part.keys.all?(Integer)
              if hash_from_multiparameter_assignment
                raise ArgumentError unless part.size == part.each_key.max
                part = klass.new(*part.sort.map(&:last))
              end

              if part.nil? && allow_nil
                mapping.each { |key, _| write_attribute(key, nil) }
                @aggregation_cache[name] = nil
              else
                mapping.each { |key, value| write_attribute(key, part.send(value)) }
                @aggregation_cache[name] = part.dup.freeze
              end
            end
          end