#409

backend/app/models/post.rb

@@ -57,7 +57,7 @@ class Post < ApplicationRecord
 
   attribute :version_no, :integer, default: 1
 
-  before_validation :normalise_url
+  before_validation :normalise_url, if: :will_save_change_to_url?
 
   validates :url, presence: true, uniqueness: true
   validates :video_ms, numericality: { only_integer: true, greater_than: 0 }, allow_nil: true
@@ -180,7 +180,7 @@ class Post < ApplicationRecord
 
     u.host = u.host.downcase if u.host
     u.path = u.path.sub(/\/\Z/, '') if u.path.present?
-    self.url = u.to_s
+    self.url = PostUrlSanitisationRule.sanitise(u.to_s)
   rescue URI::InvalidURIError
     ;
   end

backend/app/models/post_url_sanitisation_rule.rb

@@ -0,0 +1,77 @@
+class PostUrlSanitisationRule < ApplicationRecord
+  include Discard::Model
+
+  class UrlConflictError < StandardError
+    attr_reader :conflicts
+
+    def initialize(conflicts)
+      @conflicts = conflicts
+      urls = conflicts.map { _1.fetch(:url) }.uniq.join(', ')
+      super("post URL sanitisation conflicts detected for #{ urls }")
+    end
+  end
+
+  self.primary_key = :priority
+
+  default_scope -> { kept }
+
+  validates :source_pattern, presence: true, uniqueness: true
+
+  validate :source_pattern_must_be_regexp
+
+  class << self
+    def sanitise(url) =
+        rules.reduce(url.dup) { |value, (pattern, replacement)| value.sub(pattern, replacement) }
+
+    def apply!
+      rewrites = Post.order(:id).pluck(:id, :url).map do |post_id, original_url|
+        { post_id:,
+          original_url:,
+          sanitised_url: sanitise(original_url) }
+      end
+
+      conflicts = rewrites
+                      .group_by { _1.fetch(:sanitised_url) }
+                      .values
+                      .filter { _1.size > 1 }
+                      .flatten
+                      .map { { url: _1.fetch(:sanitised_url),
+                               post_id: _1.fetch(:post_id),
+                               original_url: _1.fetch(:original_url) } }
+
+      raise UrlConflictError.new(conflicts) if conflicts.present?
+
+      changed = rewrites.filter { _1.fetch(:original_url) != _1.fetch(:sanitised_url) }
+      return if changed.empty?
+
+      token = SecureRandom.hex(6)
+
+      Post.transaction do
+        changed.each do |row|
+          Post.where(id: row.fetch(:post_id))
+              .update_all(url: temporary_url_for(row.fetch(:post_id), token))
+        end
+
+        changed.each do |row|
+          Post.where(id: row.fetch(:post_id))
+              .update_all(url: row.fetch(:sanitised_url))
+        end
+      end
+    end
+
+    private
+
+    def rules = kept.order(:priority).map { |r| [Regexp.new(r.source_pattern), r.replacement] }
+
+    def temporary_url_for(post_id, token) =
+        "https://post-url-sanitising.invalid/#{ token }/#{ post_id }"
+  end
+
+  private
+
+  def source_pattern_must_be_regexp
+    Regexp.new(source_pattern)
+  rescue RegexpError
+    errors.add :source_pattern, '変な正規表現だね〜（笑）'
+  end
+end