#2 完了

backend/app/models/wiki_page.rb

@@ -0,0 +1,11 @@
+class WikiPage < ApplicationRecord
+  belongs_to :tag, optional: true
+  belongs_to :created_user, class_name: 'User', foreign_key: 'created_user_id'
+  belongs_to :updated_user, class_name: 'User', foreign_key: 'updated_user_id'
+
+  validates :title, presence: true, length: { maximum: 255 }, uniqueness: true
+
+  def gollum_path
+    "wiki/#{ title.parameterize }.md"
+  end
+end