dont recreate artist urls that havent changed

app/models/artist.rb

@@ -44,9 +44,14 @@ class Artist < ActiveRecord::Base
 
     def save_url_string
       if @url_string
-        urls.clear
+        prev = urls.map(&:url)
+        curr = @url_string.scan(/\S+/)
 
-        @url_string.scan(/\S+/).each do |url|
+        (prev - curr).each do |url|
+          urls.where(:url => url).destroy_all
+        end
+
+        (curr - prev).each do |url|
           urls.create(:url => url)
         end
       end

test/unit/artist_test.rb

@@ -124,6 +124,14 @@ class ArtistTest < ActiveSupport::TestCase
       assert_equal(["http://not.rembrandt.com/test.jpg"], artist.urls.map(&:to_s).sort)
     end
 
+    should "not delete urls that have not changed" do
+      artist = FactoryGirl.create(:artist, :name => "rembrandt", :url_string => "http://rembrandt.com/test.jpg")
+      old_url_ids = ArtistUrl.order("id").pluck(&:id)
+      artist.url_string = "http://rembrandt.com/test.jpg"
+      artist.save
+      assert_equal(old_url_ids, ArtistUrl.order("id").pluck(&:id))
+    end
+
     should "ignore pixiv.net/ and pixiv.net/img/ url matches" do
       a1 = FactoryGirl.create(:artist, :name => "yomosaka", :url_string => "http://i2.pixiv.net/img100/img/yomosaka/27618292.jpg")
       a2 = FactoryGirl.create(:artist, :name => "niwatazumi_bf", :url_string => "http://i2.pixiv.net/img16/img/niwatazumi_bf/35488864_big_p6.jpg")