Improve validation for `TopicStatusUpdate`.

app/models/topic.rb

@@ -969,7 +969,7 @@ SQL
     topic_status_update.based_on_last_post = !based_on_last_post.blank?
 
     if status_type == TopicStatusUpdate.types[:publish_to_category]
-      topic_status_update.category_id = category_id
+      topic_status_update.category = Category.find_by(category_id)
     end
 
     if topic_status_update.based_on_last_post

app/models/topic_status_update.rb

@@ -9,6 +9,8 @@ class TopicStatusUpdate < ActiveRecord::Base
   validates :execute_at, presence: true
   validates :status_type, presence: true
   validates :status_type, uniqueness: { scope: [:topic_id, :deleted_at] }
+  validates :category_id, presence: true, if: :publishing_to_category?
+
   validate :ensure_update_will_happen
 
   before_save do
@@ -93,6 +95,10 @@ class TopicStatusUpdate < ActiveRecord::Base
     def schedule_auto_publish_to_category_job(time)
       Jobs.enqueue_at(time, :publish_topic_to_category, topic_status_update_id: id)
     end
+
+    def publishing_to_category?
+      self.status_type.to_i == TopicStatusUpdate.types[:publish_to_category]
+    end
 end
 
 # == Schema Information

spec/models/topic_status_update_spec.rb

@@ -45,6 +45,34 @@ RSpec.describe TopicStatusUpdate, type: :model do
         end
       end
     end
+
+    describe '#category_id' do
+      describe 'when #status_type is publish_to_category' do
+        describe 'when #category_id is not present' do
+          it 'should not be valid' do
+            topic_status_update = Fabricate.build(:topic_status_update,
+              status_type: TopicStatusUpdate.types[:publish_to_category]
+            )
+
+            expect(topic_status_update).to_not be_valid
+            expect(topic_status_update.errors.keys).to include(:category_id)
+          end
+        end
+
+        describe 'when #category_id is present' do
+          it 'should be valid' do
+            topic_status_update = Fabricate.build(:topic_status_update,
+              status_type: TopicStatusUpdate.types[:publish_to_category],
+              category_id: Fabricate(:category).id,
+              user: Fabricate(:user),
+              topic: Fabricate(:topic)
+            )
+
+            expect(topic_status_update).to be_valid
+          end
+        end
+      end
+    end
   end
 
   context 'callbacks' do