Fixed post missing user error when create topic

app/controllers/topics_controller.rb

@@ -16,6 +16,7 @@ class TopicsController < ApplicationController
 
   def create
     @topic = Topic.new topic_params.merge(user: current_user)
+    @topic.posts.first.user = current_user
     if @topic.save
       redirect_to @topic
     else

test/controllers/topics_controller_test.rb

@@ -28,6 +28,8 @@ class TopicsControllerTest < ActionController::TestCase
     topic = Topic.last
     assert_equal 'Title', topic.title
     assert_equal 'Content', topic.posts.first.content
+    assert_not_nil topic.user
+    assert_equal topic.user, topic.posts.first.user
     assert_redirected_to topic
   end
 end