FEATURE: Event on topic merge (#6057)

app/models/topic.rb

@@ -913,6 +913,7 @@ class Topic < ActiveRecord::Base
 
     if opts[:destination_topic_id]
       post_mover.to_topic opts[:destination_topic_id]
+      DiscourseEvent.trigger(:topic_merged,  post_mover.original_topic, post_mover.destination_topic)
     elsif opts[:title]
       post_mover.to_new_topic(opts[:title], opts[:category_id], opts[:tags])
     end

spec/requests/topics_controller_spec.rb

@@ -201,6 +201,13 @@ RSpec.describe TopicsController do
           expect(result['success']).to eq(true)
           expect(result['url']).to be_present
         end
+        
+        it "triggers an event on merge" do
+          DiscourseEvent.on(:topic_merged) do |original_topic, destination_topic|
+            expect(original_topic.id).to eq(topic.id)
+            expect(destination_topic.id).to eq(dest_topic.id)
+          end
+        end
       end
 
       context 'failure' do