add achievement management

app/models/wow_achievement.rb

@@ -0,0 +1,22 @@
+# frozen_string_literal: true
+
+class WowAchievement < ApplicationRecord
+  extend Mobility
+  translates :name, :description, :reward_description
+
+  belongs_to :wow_achievement_category, optional: true
+  belongs_to :wow_achievement_criterium, optional: true
+
+  belongs_to :prerequisite_achievement, class_name: 'WowAchievement', optional: true
+  has_one :next_achievement,
+          class_name: 'WowAchievement',
+          foreign_key: 'prerequisite_achievement_id',
+          dependent: :nullify,
+          inverse_of: :prerequisite_achievement
+
+  has_many :completed_wow_achievements, dependent: :destroy
+  has_many :wow_characters, through: :completed_wow_achievements
+
+  validates :name, presence: true
+  validates :achievement_id, presence: true, uniqueness: true
+end