子モデルのバリデーションエラーをcascadeするバリデータを追加する

backend/app/validators/cascade_validator.rb

@@ -0,0 +1,79 @@
+# frozen_string_literal: true
+
+class CascadeValidator < ActiveModel::EachValidator
+  def validate_each(record, attribute, value)
+    Validator
+      .call(parent: self, record: record, attribute: attribute, value: value)
+  end
+
+  class Validator
+    private_class_method :new
+
+    class << self
+      def call(parent:, record:, attribute:, value:)
+        new(
+          parent: parent, record: record, attribute: attribute, value: value
+        ).call
+      end
+    end
+
+    def initialize(parent:, record:, attribute:, value:)
+      @parent = parent
+      @record = record
+      @attribute = attribute
+      @value = value
+    end
+
+    def call
+      return if value.nil? || valid?
+
+      add_error!
+    end
+
+    private
+
+    attr_reader :parent
+    attr_reader :record
+    attr_reader :attribute
+    attr_reader :value
+
+    delegate :options, to: :parent
+
+    def associations
+      @associations ||= \
+        if value.class.include?(Enumerable) && !value.is_a?(Struct)
+          value
+        else
+          [value]
+        end.select { _1.respond_to?(:valid?) }
+    end
+
+    def valid?
+      invalid_associations.empty?
+    end
+
+    def invalid_associations
+      @invalid_associations ||= \
+        associations.select { _1.invalid?(context) }
+    end
+
+    def invalid_message
+      invalid_associations
+        .map { _1.errors.full_messages.to_sentence }
+        .to_sentence
+    end
+
+    def context = options.try(:[], :context)
+
+    def add_error!
+      record
+        .errors
+        .add(
+          attribute,
+          :invalid_association,
+          association: invalid_message
+        )
+    end
+  end
+  private_constant :Validator
+end

backend/spec/validators/cascade_validator_spec.rb

@@ -0,0 +1,109 @@
+# frozen_string_literal: true
+
+RSpec.describe CascadeValidator do
+  let(:child_klass) do
+    child_test_class = Class.new do
+      attr_accessor :name, :age
+
+      include ActiveModel::Validations
+      validates :name, presence: true
+      validates :age, presence: true, inclusion: { in: 18..65 }
+    end
+
+    stub_const('Child', child_test_class)
+  end
+
+  context '対象が配列ではないとき' do
+    let(:klass) do
+      test_class = Class.new do
+        attr_accessor :child
+
+        include ActiveModel::Validations
+
+        validates :child, cascade: true
+      end
+
+      stub_const('TestClass', test_class)
+    end
+
+    let(:model) do
+      klass.new.tap do |instance|
+        instance.child = child_klass.new
+      end
+    end
+
+    it 'validatesに渡されたメソッドに対してvalid?による検証を行う' do
+      aggregate_failures do
+        expect(model.valid?).to be false
+        model.child.name = 'hoge'
+        model.child.age = 18
+        expect(model.valid?).to be true
+        model.child.age = 11
+        expect(model.valid?).to be false
+      end
+    end
+  end
+
+  context 'valueがnilを返却する時' do
+    let(:klass) do
+      test_class = Class.new do
+        attr_accessor :child
+
+        include ActiveModel::Validations
+
+        validates :child, cascade: true
+      end
+
+      stub_const('TestClass', test_class)
+    end
+
+    let(:model) do
+      klass.new.tap do |instance|
+        instance.child = child_klass.new
+      end
+    end
+
+    before { model.child = nil }
+
+    it 'cascadeは実行されないため評価はスキップされる。' do
+      expect(model.valid?).to be true
+    end
+  end
+
+  context '対象が配列のとき' do
+    let(:klass) do
+      children_test_class = Class.new do
+        attr_accessor :children
+
+        include ActiveModel::Validations
+
+        validates :children, cascade: true
+      end
+
+      stub_const('TestClass', children_test_class)
+    end
+
+    let(:model) do
+      klass.new.tap do |instance|
+        instance.children = Array.new(3) { child_klass.new }
+      end
+    end
+
+    it 'validatesに渡されたメソッドに対してvalid?による検証を行う' do
+      aggregate_failures do
+        expect(model.valid?).to be false
+        model.children.first.name = 'hoge'
+        model.children.first.age = 18
+        expect(model.valid?).to be false
+        model.children.second.name = 'hoge'
+        model.children.second.age = 18
+        expect(model.valid?).to be false
+        model.children.third.name = 'hoge'
+        model.children.third.age = 18
+        expect(model.valid?).to be true
+        model.children.second.age = 11
+        expect(model.valid?).to be false
+      end
+    end
+  end
+end