dtext links: add table for tracking links between wikis.

Add a dtext_links table for tracking links between wiki pages. This is
to allow for broken link detection and "what links here" searches, among
other uses.

app/models/dtext_link.rb

@@ -0,0 +1,27 @@
+class DtextLink < ApplicationRecord
+  belongs_to :model, polymorphic: true
+  enum link_type: [:wiki_link, :external_link]
+
+  before_validation :normalize_link_target
+  validates :link_target, uniqueness: { scope: [:model_type, :model_id] }
+
+  def self.new_from_dtext(dtext)
+    links = []
+
+    links += DText.parse_wiki_titles(dtext).map do |link|
+      DtextLink.new(link_type: :wiki_link, link_target: link)
+    end
+
+    links += DText.parse_external_links(dtext).map do |link|
+      DtextLink.new(link_type: :external_link, link_target: link)
+    end
+
+    links
+  end
+
+  def normalize_link_target
+    if wiki_link?
+      self.link_target = WikiPage.normalize_title(link_target)
+    end
+  end
+end

app/models/wiki_page.rb

@@ -5,6 +5,7 @@ class WikiPage < ApplicationRecord
 
   before_save :normalize_title
   before_save :normalize_other_names
+  before_save :update_dtext_links, if: :dtext_links_changed?
   after_save :create_version
   validates_uniqueness_of :title, :case_sensitive => false
   validates_presence_of :title
@@ -19,6 +20,7 @@ class WikiPage < ApplicationRecord
   has_one :tag, :foreign_key => "name", :primary_key => "title"
   has_one :artist, -> {where(:is_active => true)}, :foreign_key => "name", :primary_key => "title"
   has_many :versions, -> {order("wiki_page_versions.id ASC")}, :class_name => "WikiPageVersion", :dependent => :destroy
+  has_many :dtext_links, as: :model, dependent: :destroy
 
   api_attributes including: [:creator_name, :category_name]
 
@@ -197,6 +199,14 @@ class WikiPage < ApplicationRecord
     end
   end
 
+  def dtext_links_changed?
+    body_changed? && DText.dtext_links_differ?(body, body_was)
+  end
+
+  def update_dtext_links
+    self.dtext_links = DtextLink.new_from_dtext(body)
+  end
+
   def post_set
     @post_set ||= PostSets::WikiPage.new(title, 1, 4)
   end