aliases: refactor tag moving code.

* Factor out the code for moving tags from tag aliases to a separate
  TagMover class.

* When aliasing two tags that have conflicting wikis, merge the old wiki
  into the new one instead of failing with an error. Merge the other names
  fields, replace the old wiki body with a message linking to the new
  wiki, and mark the old wiki as deleted.

* When aliasing two tags that have conflicting artist entries, merge the
  old artist into the new one instead of silently ignore the conflict.
  Merge the group name, other names, and urls fields, and mark the old
  artist as deleted.

* When two tags have conflicting wikis or artist entries, but the old
  wiki or artist entry is deleted, then just ignore the old wiki or
  artist and don't try to merge it.

* Fix it so that when saved searches are rewritten, we rewrite negated
  searches too.

app/logical/concerns/searchable.rb

@@ -57,12 +57,12 @@ module Searchable
 
   # https://www.postgresql.org/docs/current/static/functions-matching.html#FUNCTIONS-POSIX-REGEXP
   # "(?e)" means force use of ERE syntax; see sections 9.7.3.1 and 9.7.3.4.
-  def where_regex(attr, value)
-    where("#{qualified_column_for(attr)} ~ ?", "(?e)" + value)
+  def where_regex(attr, value, flags: "e")
+    where("#{qualified_column_for(attr)} ~ ?", "(?#{flags})" + value)
   end
 
-  def where_not_regex(attr, value)
-    where.not("#{qualified_column_for(attr)} ~ ?", "(?e)" + value)
+  def where_not_regex(attr, value, flags: "e")
+    where.not("#{qualified_column_for(attr)} ~ ?", "(?#{flags})" + value)
   end
 
   def where_inet_matches(attr, value)

app/logical/tag_mover.rb

@@ -0,0 +1,108 @@
+class TagMover
+  attr_reader :old_tag, :new_tag, :user
+
+  def initialize(old_name, new_name, user: User.system)
+    @old_tag = Tag.find_or_create_by_name(old_name)
+    @new_tag = Tag.find_or_create_by_name(new_name)
+    @user = user
+  end
+
+  def move!
+    CurrentUser.scoped(user) do
+      move_tag_category!
+      move_artist!
+      move_wiki!
+      move_saved_searches!
+      move_posts!
+    end
+  end
+
+  def move_tag_category!
+    if old_tag.general? && !new_tag.general?
+      old_tag.update!(category: new_tag.category)
+    elsif new_tag.general? && !old_tag.general?
+      new_tag.update!(category: old_tag.category)
+    end
+  end
+
+  def move_artist!
+    return unless old_tag.artist? && old_artist.present? && !old_artist.is_deleted?
+
+    if new_artist.nil?
+      old_artist.update!(name: new_tag.name)
+    else
+      merge_artists!
+    end
+  end
+
+  def move_wiki!
+    return unless old_wiki.present? && !old_wiki.is_deleted?
+
+    if new_wiki.nil?
+      old_wiki.update!(title: new_tag.name)
+    else
+      merge_wikis!
+    end
+  end
+
+  def move_posts!
+    Post.raw_tag_match(old_tag.name).find_each do |post|
+      post.lock!
+      post.remove_tag(old_tag.name)
+      post.add_tag(new_tag.name)
+      post.save!
+    end
+  end
+
+  def move_saved_searches!
+    SavedSearch.rewrite_queries!(old_tag.name, new_tag.name)
+  end
+
+  def merge_artists!
+    old_artist.lock!
+    new_artist.lock!
+
+    new_artist.other_names += old_artist.other_names
+    new_artist.other_names += [old_artist.name]
+    new_artist.group_name = old_artist.group_name unless new_artist.group_name.present?
+    new_artist.url_string += "\n" + old_artist.url_string
+    new_artist.is_deleted = false
+    new_artist.save!
+
+    old_artist.other_names = [new_artist.name]
+    old_artist.group_name = ""
+    old_artist.url_string = ""
+    old_artist.is_deleted = true
+    old_artist.save!
+  end
+
+  def merge_wikis!
+    old_wiki.lock!
+    new_wiki.lock!
+
+    new_wiki.other_names += old_wiki.other_names
+    new_wiki.is_deleted = false
+    new_wiki.save!
+
+    old_wiki.body = "This tag has been moved to [[#{new_wiki.title}]]."
+    old_wiki.other_names = []
+    old_wiki.is_deleted = true
+    old_wiki.save!
+  end
+
+  def old_wiki
+    old_tag.wiki_page
+  end
+
+  def new_wiki
+    new_tag.wiki_page
+  end
+
+  def old_artist
+    old_tag.artist
+  end
+
+  def new_artist
+    new_tag.artist
+  end
+end