btrc-hub/backend/app/models/material.rb

class Material < ApplicationRecord
  include MyDiscard

  default_scope -> { kept }

  belongs_to :parent, class_name: 'Material', optional: true
  has_many :children, class_name: 'Material', foreign_key: :parent_id, dependent: :nullify

  belongs_to :tag, optional: true
  belongs_to :created_by_user, class_name: 'User', optional: true
  belongs_to :updated_by_user, class_name: 'User', optional: true

  has_one_attached :file, dependent: :purge

  validates :tag_id, presence: true, uniqueness: true

  validate :file_must_be_attached
  validate :tag_must_be_material_category

  def content_type
    return nil unless file&.attached?

    file.blob.content_type
  end

  private

  def file_must_be_attached
    return if url.present? || file.attached?

    errors.add(:url, 'URL かファイルのどちらかは必須です．')
  end

  def tag_must_be_material_category
    return if tag.blank? || tag.character? || tag.material?

    errors.add(:tag, '素材カテゴリのタグを指定してください．')
  end
end