Keep HTML attachment slugs between versions

app/models/html_attachment.rb

@@ -40,4 +40,8 @@ def url(options = {})
   def extracted_text
     Govspeak::Document.new(body).to_text
   end
+
+  def should_generate_new_friendly_id?
+    slug.nil? || attachable.nil? || !attachable.document.published?
+  end
 end

test/unit/html_attachment_test.rb

@@ -9,6 +9,33 @@ class HtmlAttachmentTest < ActiveSupport::TestCase
   end
 
   test "slug is copied from previous edition's attachment" do
-    skip 'TODO'
+    attachment = create(:html_attachment, title: "an-html-attachment")
+    edition = create(:published_publication, attachments: [attachment])
+    draft = edition.create_draft(create(:policy_writer))
+
+    assert_equal "an-html-attachment", draft.attachments.first.slug
+  end
+
+  test "slug is updated when the title is changed if edition is unpublished" do
+    attachment = create(:html_attachment, title: "an-html-attachment")
+    edition = create(:draft_publication, attachments: [attachment])
+
+    attachment.title = "a-new-title"
+    attachment.save
+    attachment.reload
+
+    assert_equal "a-new-title", attachment.slug
+  end
+
+  test "slug is not updated when the title is changed if edition is published" do
+    edition = create(:published_publication, attachments: [create(:html_attachment, title: "an-html-attachment")])
+    draft = edition.create_draft(create(:policy_writer))
+    attachment = draft.attachments.first
+
+    attachment.title = "a-new-title"
+    attachment.save
+    attachment.reload
+
+    assert_equal "an-html-attachment", attachment.slug
   end
 end