ActiveRecord::NestedAttributes

ActiveRecord::NestedAttributes

nest attributes的实现部分，也就是嵌套数据部分：

# frozen_string_literal: true

require "active_support/core_ext/hash/except"
require "active_support/core_ext/module/redefine_method"
require "active_support/core_ext/object/try"
require "active_support/core_ext/hash/indifferent_access"

module ActiveRecord
  module NestedAttributes #:nodoc:
    class TooManyRecords < ActiveRecordError
    end

    extend ActiveSupport::Concern

    included do
      class_attribute :nested_attributes_options, instance_writer: false, default: {}
    end

    module ClassMethods
      REJECT_ALL_BLANK_PROC = proc { |attributes| attributes.all? { |key, value| key == "_destroy" || value.blank? } }

      def accepts_nested_attributes_for(*attr_names)
        options = { allow_destroy: false, update_only: false }
        options.update(attr_names.extract_options!)
        options.assert_valid_keys(:allow_destroy, :reject_if, :limit, :update_only)
        options[:reject_if] = REJECT_ALL_BLANK_PROC if options[:reject_if] == :all_blank

        attr_names.each do |association_name|
          if reflection = _reflect_on_association(association_name)
            reflection.autosave = true
            define_autosave_validation_callbacks(reflection)

            nested_attributes_options = self.nested_attributes_options.dup
            nested_attributes_options[association_name.to_sym] = options
            self.nested_attributes_options = nested_attributes_options

            type = (reflection.collection? ? :collection : :one_to_one)
            generate_association_writer(association_name, type)
          else
            raise ArgumentError, "No association found for name `#{association_name}'. Has it been defined yet?"
          end
        end
      end

      private

        def generate_association_writer(association_name, type)
          generated_association_methods.module_eval <<-eoruby, __FILE__, __LINE__ + 1
            silence_redefinition_of_method :#{association_name}_attributes=
            def #{association_name}_attributes=(attributes)
              assign_nested_attributes_for_#{type}_association(:#{association_name}, attributes)
            end
          eoruby
        end
    end

    def _destroy
      marked_for_destruction?
    end

    private

      UNASSIGNABLE_KEYS = %w( id _destroy )

      def assign_nested_attributes_for_one_to_one_association(association_name, attributes)
        options = nested_attributes_options[association_name]
        if attributes.respond_to?(:permitted?)
          attributes = attributes.to_h
        end
        attributes = attributes.with_indifferent_access
        existing_record = send(association_name)

        if (options[:update_only] || !attributes["id"].blank?) && existing_record &&
            (options[:update_only] || existing_record.id.to_s == attributes["id"].to_s)
          assign_to_or_mark_for_destruction(existing_record, attributes, options[:allow_destroy]) unless call_reject_if(association_name, attributes)

        elsif attributes["id"].present?
          raise_nested_attributes_record_not_found!(association_name, attributes["id"])

        elsif !reject_new_record?(association_name, attributes)
          assignable_attributes = attributes.except(*UNASSIGNABLE_KEYS)

          if existing_record && existing_record.new_record?
            existing_record.assign_attributes(assignable_attributes)
            association(association_name).initialize_attributes(existing_record)
          else
            method = :"build_#{association_name}"
            if respond_to?(method)
              send(method, assignable_attributes)
            else
              raise ArgumentError, "Cannot build association `#{association_name}'. Are you trying to build a polymorphic one-to-one association?"
            end
          end
        end
      end

      def assign_nested_attributes_for_collection_association(association_name, attributes_collection)
        options = nested_attributes_options[association_name]
        if attributes_collection.respond_to?(:permitted?)
          attributes_collection = attributes_collection.to_h
        end

        unless attributes_collection.is_a?(Hash) || attributes_collection.is_a?(Array)
          raise ArgumentError, "Hash or Array expected for attribute `#{association_name}`, got #{attributes_collection.class.name} (#{attributes_collection.inspect})"
        end

        check_record_limit!(options[:limit], attributes_collection)

        if attributes_collection.is_a? Hash
          keys = attributes_collection.keys
          attributes_collection = if keys.include?("id") || keys.include?(:id)
            [attributes_collection]
          else
            attributes_collection.values
          end
        end

        association = association(association_name)

        existing_records = if association.loaded?
          association.target
        else
          attribute_ids = attributes_collection.map { |a| a["id"] || a[:id] }.compact
          attribute_ids.empty? ? [] : association.scope.where(association.klass.primary_key => attribute_ids)
        end

        attributes_collection.each do |attributes|
          if attributes.respond_to?(:permitted?)
            attributes = attributes.to_h
          end
          attributes = attributes.with_indifferent_access

          if attributes["id"].blank?
            unless reject_new_record?(association_name, attributes)
              association.reader.build(attributes.except(*UNASSIGNABLE_KEYS))
            end
          elsif existing_record = existing_records.detect { |record| record.id.to_s == attributes["id"].to_s }
            unless call_reject_if(association_name, attributes)
              # Make sure we are operating on the actual object which is in the association's
              # proxy_target array (either by finding it, or adding it if not found)
              # Take into account that the proxy_target may have changed due to callbacks
              target_record = association.target.detect { |record| record.id.to_s == attributes["id"].to_s }
              if target_record
                existing_record = target_record
              else
                association.add_to_target(existing_record, :skip_callbacks)
              end

              assign_to_or_mark_for_destruction(existing_record, attributes, options[:allow_destroy])
            end
          else
            raise_nested_attributes_record_not_found!(association_name, attributes["id"])
          end
        end
      end

      # Takes in a limit and checks if the attributes_collection has too many
      # records. It accepts limit in the form of symbol, proc, or
      # number-like object (anything that can be compared with an integer).
      #
      # Raises TooManyRecords error if the attributes_collection is
      # larger than the limit.
      def check_record_limit!(limit, attributes_collection)
        if limit
          limit = \
            case limit
            when Symbol
              send(limit)
            when Proc
              limit.call
            else
              limit
            end

          if limit && attributes_collection.size > limit
            raise TooManyRecords, "Maximum #{limit} records are allowed. Got #{attributes_collection.size} records instead."
          end
        end
      end

      # Updates a record with the +attributes+ or marks it for destruction if
      # +allow_destroy+ is +true+ and has_destroy_flag? returns +true+.
      def assign_to_or_mark_for_destruction(record, attributes, allow_destroy)
        record.assign_attributes(attributes.except(*UNASSIGNABLE_KEYS))
        record.mark_for_destruction if has_destroy_flag?(attributes) && allow_destroy
      end

      # Determines if a hash contains a truthy _destroy key.
      def has_destroy_flag?(hash)
        Type::Boolean.new.cast(hash["_destroy"])
      end

      # Determines if a new record should be rejected by checking
      # has_destroy_flag? or if a :reject_if proc exists for this
      # association and evaluates to +true+.
      def reject_new_record?(association_name, attributes)
        will_be_destroyed?(association_name, attributes) || call_reject_if(association_name, attributes)
      end

      # Determines if a record with the particular +attributes+ should be
      # rejected by calling the reject_if Symbol or Proc (if defined).
      # The reject_if option is defined by +accepts_nested_attributes_for+.
      #
      # Returns false if there is a +destroy_flag+ on the attributes.
      def call_reject_if(association_name, attributes)
        return false if will_be_destroyed?(association_name, attributes)

        case callback = nested_attributes_options[association_name][:reject_if]
        when Symbol
          method(callback).arity == 0 ? send(callback) : send(callback, attributes)
        when Proc
          callback.call(attributes)
        end
      end

      # Only take into account the destroy flag if :allow_destroy is true
      def will_be_destroyed?(association_name, attributes)
        allow_destroy?(association_name) && has_destroy_flag?(attributes)
      end

      def allow_destroy?(association_name)
        nested_attributes_options[association_name][:allow_destroy]
      end

      def raise_nested_attributes_record_not_found!(association_name, record_id)
        model = self.class._reflect_on_association(association_name).klass.name
        raise RecordNotFound.new("Couldn't find #{model} with ID=#{record_id} for #{self.class.name} with ID=#{id}",
                                 model, "id", record_id)
      end
  end
end

主要部分为module ClassMethods里实现的accepts_nested_attributes_for,首先是保存options到类实例变量里，设置reflection为autosave,定义validations的回调，生成相应的writer方法,以"#{association_name}_attributes="这样的名称。