FIX: N1 issues for bookmark list (#9236)

* Preload custom fields for BookmarkQuery and add preload callback. Copy TopicQuery preload methodology to allow plugins to preload data for the BookmarkQuery. This fixes assigned plugin custom fields N1
* Include topic tags in initial query to avoid tags N1

Related: discourse/discourse-assign#63

spec/lib/bookmark_query_spec.rb

@@ -0,0 +1,51 @@
+# frozen_string_literal: true
+
+require 'rails_helper'
+
+RSpec.describe BookmarkQuery do
+  fab!(:user) { Fabricate(:user) }
+  fab!(:bookmark1) { Fabricate(:bookmark, user: user) }
+  fab!(:bookmark2) { Fabricate(:bookmark, user: user) }
+  let(:params) { {} }
+
+  subject { described_class.new(user, params) }
+
+  describe "#list_all" do
+    it "returns all the bookmarks for a user" do
+      expect(subject.list_all.count).to eq(2)
+    end
+
+    it "runs the on_preload block provided passing in bookmarks" do
+      preloaded_bookmarks = []
+      BookmarkQuery.on_preload do |bookmarks, bq|
+        (preloaded_bookmarks << bookmarks).flatten
+      end
+      subject.list_all
+      expect(preloaded_bookmarks.any?).to eq(true)
+    end
+
+    context "when the limit param is provided" do
+      let(:params) { { limit: 1 } }
+      it "is respected" do
+        expect(subject.list_all.count).to eq(1)
+      end
+    end
+
+    context "when there are topic custom fields to preload" do
+      before do
+        TopicCustomField.create(
+          topic_id: bookmark1.topic.id, name: 'test_field', value: 'test'
+        )
+        BookmarkQuery.preloaded_custom_fields << "test_field"
+      end
+      it "preloads them" do
+        Topic.expects(:preload_custom_fields)
+        expect(
+          subject.list_all.find do |b|
+            b.topic_id = bookmark1.topic_id
+          end.topic.custom_fields['test_field']
+        ).not_to eq(nil)
+      end
+    end
+  end
+end