preload email correctly (not backwards compat with stable)

plugin.rb

@@ -263,12 +263,13 @@ SQL
       allowed_access = SiteSetting.assigns_public || is_staff
 
       if allowed_access && topics.length > 0
-        users = User.where("id in (
+        users = User.where("users.id in (
               SELECT value::int
               FROM topic_custom_fields
               WHERE name = 'assigned_to_id' AND topic_id IN (?)
         )", topics.map(&:id))
-        .select(:id, :email, :username, :uploaded_avatar_id)
+          .joins('join user_emails on user_emails.user_id = users.id AND user_emails.primary')
+          .select(:id, 'user_emails.email', :username, :uploaded_avatar_id)
 
         map = {}
         users.each { |u| map[u.id] = u }
@@ -389,12 +390,10 @@ SQL
 
     Discourse::Application.routes.append do
       mount ::DiscourseAssign::Engine, at: "/assign"
-      get "topics/private-messages-assigned/:username" => "list#private_messages_assigned",
-        as: "topics_private_messages_assigned", constraints: {username: /[\w.\-]+?/}
+      get "topics/private-messages-assigned/:username" => "list#private_messages_assigned", as: "topics_private_messages_assigned", constraints: { username: /[\w.\-]+?/ }
     end
   end
 
-
   on(:post_created) do |post|
     ::TopicAssigner.auto_assign(post, force: true)
   end

spec/integration/assign_spec.rb

@@ -0,0 +1,11 @@
+require 'rails_helper'
+
+describe 'integration tests' do
+  it 'preloads data in topic list' do
+    admin = Fabricate(:admin)
+    post = create_post
+    list = TopicList.new("latest", admin, [post.topic])
+    TopicList.preload([post.topic], list)
+    # should not explode for now
+  end
+end