関連の持ち主が同一人物であることをテスト

association_owner_validator.rb

# 使い方
# validates_with(
#   AssociationOwnerValidator,
#   associations: {
#     sub_item: :user
#   },
#   owner: -> { profile_image.user },
#   if: %i[sub_item profile_image]
# )
class AssociationOwnerValidator < ActiveModel::Validator
  def validate(record)
    owner = owner_association(record)
    return unless owner

    options.fetch(:associations).each do |reflection_name, owner_reflection_name|
      reflection = record.class.reflections[reflection_name.to_s]
      owner_reflection = find_owner_reflection(record, reflection, owner_reflection_name)

      if reflection.collection?
        validate_collection(owner, record, reflection, owner_reflection)
      else
        validate_singlar(owner, record, reflection, owner_reflection)
      end
    end
  end

  private

  def owner_association(record)
    owner = options.fetch(:owner)

    if owner.respond_to?(:call)
      record.instance_exec(&owner)
    elsif owner.respond_to?(:to_sym)
      record.send(owner)
    else
      owner
    end
  end

  def find_owner_reflection(record, reflection, owner_reflection_name)
    if reflection.polymorphic?
      record.association(reflection.name).klass.reflections[owner_reflection_name.to_s]
    else
      reflection.klass.reflections[owner_reflection_name.to_s]
    end
  end

  def validate_singlar(owner, record, reflection, owner_reflection)
    association = record.public_send(reflection.name)
    record.errors.add(reflection.name.to_sym, :invalid_association) if invalid?(owner, association, owner_reflection)
  end

  def validate_collection(owner, record, reflection, owner_reflection)
    record.public_send(reflection.name).each do |association|
      record.errors.add(reflection.name.to_sym, :invalid_association) if invalid?(owner, association, owner_reflection)
    end
  end

  def invalid?(owner, record, owner_reflection)
    record.public_send(owner_reflection.name) != owner
  end
end