Unscope post + topic relation for bookmark

* we want to be able to get these records for the bookmark
  even if they are trashed, for serialization for the bookmark
  list

spec/serializers/user_bookmark_serializer_spec.rb

@@ -0,0 +1,35 @@
+# frozen_string_literal: true
+
+require 'rails_helper'
+
+RSpec.describe UserBookmarkSerializer do
+  let(:bookmark) do
+    Fabricate(:bookmark)
+    Bookmark.all.includes(post: :user).includes(:topic).last
+  end
+
+  subject { described_class.new(bookmark) }
+
+  context "when the topic is deleted" do
+    before do
+      bookmark.topic.trash!
+      bookmark.reload
+    end
+    it "still returns the topic title because the relationship is unscoped" do
+      expect(subject.title).not_to eq(nil)
+    end
+  end
+
+  context "when the post is deleted" do
+    before do
+      bookmark.post.trash!
+      bookmark.reload
+    end
+    it "still returns the post number because the relationship is unscoped" do
+      expect(subject.linked_post_number).not_to eq(nil)
+    end
+    it "still returns the post username" do
+      expect(subject.username).not_to eq(nil)
+    end
+  end
+end