user upgrades: add upgrade code system.

Add a system for upgrading accounts using upgrade codes. Users purchase
an upgrade code off-site then redeem it on-site to upgrade their account
to Gold. Upgrade codes are randomly pre-generated and are one time use
only. Codes have enough randomness that guessing a code is infeasible.

app/models/upgrade_code.rb

@@ -0,0 +1,62 @@
+# frozen_string_literal: true
+
+# A code that can be redeemed for a Gold account. Codes are pre-generated and
+# one time use only. Upgrade codes are sold in the Danbooru store.
+class UpgradeCode < ApplicationRecord
+  class InvalidCodeError < StandardError; end
+  class RedeemedCodeError < StandardError; end
+  class AlreadyUpgradedError < StandardError; end
+
+  UPGRADE_CODE_LENGTH = 8
+
+  attribute :code, default: -> { UpgradeCode.generate_code }
+  attribute :status, default: :unsold
+
+  belongs_to :creator, class_name: "User"
+  belongs_to :redeemer, class_name: "User", optional: true
+  belongs_to :user_upgrade, optional: true
+
+  enum status: {
+    unsold: 0,
+    unredeemed: 100,
+    redeemed: 200,
+  }
+
+  def self.visible(user)
+    if user.is_owner?
+      all
+    else
+      where(redeemer: user).or(where(creator: user))
+    end
+  end
+
+  def self.search(params)
+    q = search_attributes(params, :id, :created_at, :updated_at, :code, :status, :creator, :redeemer, :user_upgrade)
+    q.apply_default_order(params)
+  end
+
+  def self.generate_code
+    SecureRandom.send(:choose, [*"0".."9", *"A".."Z", *"a".."z"], UPGRADE_CODE_LENGTH) # base62
+  end
+
+  def self.redeem!(code:, redeemer:)
+    upgrade_code = UpgradeCode.find_by(code: code)
+    raise InvalidCodeError, "This upgrade code is invalid" if upgrade_code.nil?
+
+    upgrade_code.redeem!(redeemer)
+  end
+
+  def redeem!(redeemer)
+    transaction do
+      raise RedeemedCodeError, "This upgrade code has already been used" if redeemed?
+      raise AlreadyUpgradedError, "Your account is already Gold or higher" if redeemer.is_gold?
+
+      user_upgrade = UserUpgrade.create!(recipient: redeemer, purchaser: redeemer, status: "processing", upgrade_type: "gold", payment_processor: "upgrade_code")
+      user_upgrade.process_upgrade!("paid")
+
+      update!(status: :redeemed, redeemer: redeemer, user_upgrade: user_upgrade)
+
+      self
+    end
+  end
+end

app/models/user_upgrade.rb

@@ -22,6 +22,8 @@ class UserUpgrade < ApplicationRecord
   enum payment_processor: {
     stripe: 0,
     authorize_net: 100,
+    shopify: 200,
+    upgrade_code: 300,
   }
 
   scope :gifted, -> { where("recipient_id != purchaser_id") }
@@ -113,7 +115,7 @@ class UserUpgrade < ApplicationRecord
   end
 
   concerning :UpgradeMethods do
-    def process_upgrade!(payment_status)
+    def process_upgrade!(payment_status = "paid")
       recipient.with_lock do
         return unless pending? || processing?
 
@@ -174,6 +176,8 @@ class UserUpgrade < ApplicationRecord
         PaymentTransaction::Stripe.new(self)
       in "authorize_net"
         PaymentTransaction::AuthorizeNet.new(self)
+      in "shopify"
+        PaymentTransaction::Shopify.new(self)
       end
     end